feat(fast-inbox): same-block L1-to-L2 message consumption for non-first blocks (A-1432)

## What

Makes every block's `BlockConstantData.l1_to_l2_tree_snapshot` the **post-bundle** snapshot for that block — first block or not — so a public/AVM tx in block N can read the L1-to-L2 messages block N inserts (same-block consumption), instead of only from block N+1.

Previously the non-first tx-carrying variants (`block_root`, `block_root_single_tx`) read their start snapshot from `constants.l1_to_l2_tree_snapshot` and never asserted the post-bundle root, giving next-block visibility for mid-checkpoint insertions. Now they:

- take the start L1-to-L2 snapshot as a **witness input** (`previous_l1_to_l2`), pinned by block-merge continuity (`right.start_state == left.end_state`) to the previous block's end state — the checkpoint root forces the leftmost block to be a first-block variant, so every non-first block has a left neighbour pinning its start;
- append their bundle and assert `validate_l1_to_l2_tree_snapshot_in_constants(constants, new_l1_to_l2)`, exactly like the first-block variants.

The tx-less variants (`block_root_empty_tx_first`, `block_root_msgs_only`) carry no tx constants to read against and already witness their start snapshot, so they are unchanged.

## Soundness

The witnessed start snapshot cannot be forged: block-merge / checkpoint-root continuity asserts `right.start_state == left.end_state`, and the checkpoint root asserts the leftmost block is a first-block variant (whose start is pinned to the previous checkpoint). The start is therefore anchored, and the new assert pins the tx-constants snapshot to the computed post-bundle root. The start is **not** derived from "constants minus the bundle" (not expressible).

## Bit-identical pre-flip (public inputs)

Non-first bundles are empty today, so `new_l1_to_l2 == start == today's constants value`; the new assert passes with unchanged values and the block-root **public inputs / state are unchanged**. The circuit constraints, private-input ABI, proof, and VK do change (hence the regen below). This is a restructure-now / flip-minimal change.

## Flip follow-on (not in this PR)

The recursive pinning of the witnessed `previous_l1_to_l2` is already exercised by `block_merge::tests::consecutive_block_rollups_tests::non_consecutive_l1_to_l2_message_tree_snapshots` (a right block whose `start_state.l1_to_l2_message_tree` — i.e. this witness — doesn't match the left block's end is rejected on continuity).

One flip requirement this change surfaces: today the prover gives every non-first block the checkpoint's post-first-block snapshot as its `previousL1ToL2` (`block-proving-state.ts`), which is correct only while non-first bundles are empty. Once non-first blocks insert their own bundles (A-1384), the prover must instead feed block N+1 the end snapshot of block N plus the matching frontier hint. Flagged for the flip plan (A-1384).

## Tests

Adds negative nargo tests: a non-first block (two-tx and single-tx) whose `constants.l1_to_l2_tree_snapshot` differs from its post-bundle root must fail. Red/green verified locally — with the new asserts removed the four negative tests report "Test passed when it should have failed"; restored, they pass. `rollup_lib` block_root suite green (63 tests), block_merge (45), checkpoint_root structure tests (17).

## TS wiring

Mirrors the new `previous_l1_to_l2` field through `stdlib` (`BlockRootRollupPrivateInputs` / `BlockRootSingleTxRollupPrivateInputs` serialization + factory), the Noir ABI conversion (`server.ts`), and the prover-client block-root input builders (`block-proving-state.ts`), which pass the block's `lastL1ToL2MessageTreeSnapshot` (for non-first blocks, the checkpoint's post-first-block snapshot). `stdlib` builds and its serialization test passes.

## Artifact regen (rides CI / mainframe)

This changes the `block_root` and `block_root_single_tx` circuit ABIs, so it invalidates their VKs, the checkpoint-root/block-root `Prover.toml` sample inputs, and the generated `noir-protocol-circuits-types/src/types/index.ts`. These are **not** regenerated locally (`bb write_vk` OOMs on large circuits on dev boxes; the generated `index.ts` also depends on recompiled circuit artifacts). They ride the seeded-S3-cache mainframe/CI flow, consistent with the rest of the Fast Inbox stack.

## Stack

Stacked on `spl/a-1427-inbox-parity`; the L1 PRs (#24771, #24773) are rebased on top of this. Part of the Fast Inbox stack (umbrella #24774).

Fixes A-1432


Replaces #24781.

Author: spalladino

noir-projects/fnd/noir-protocol-circuits/crates/rollup-block-root-single-tx/Prover.toml

@@ -39,7 +39,7 @@ l1_to_l2_message_frontier_hint = [
 ]
 new_archive_sibling_path = [
   "0x0000000000000000000000000000000000000000000000000000000000000000",
-  "0x2fdbd1b87ca82784e3c3d2871efa66b2a61337fcb13897a786c67fbafb377149",
+  "0x2f559d93de6d3e8a6289d45995351f749463bcee0ee677456f8ee19347c27114",
   "0x14e4b977b2203b70e6ee1c2456eb7114d090fe4b907f631eecd0919fed432e7d",
   "0x30105bad22ddcc508b739b7c9ad87a561c569ff5cb0098a853c1c4ac21b7a037",
   "0x1e20ad4181460cbfdc74ca773502c59b890f184efe300ebad895956d318422da",
@@ -561,12 +561,12 @@ new_archive_sibling_path = [
     accumulated_mana_used = "0x0000000000000000000000000000000000000000000000000000000000000000"
 
       [inputs.previous_rollup.public_inputs.constants]
-      vk_tree_root = "0x0a178ec6fe216bfa6e2d25089ce97f9ff6b5c5701d2bce2a1fdf11e6dc351e3c"
+      vk_tree_root = "0x1d51e7bb66dda198c072b182a8e4809fc6614d677c914af2afa9c5376f16f06d"
       protocol_contracts_hash = "0x0727efc9473643b7abbe3c57df72d68e86b244b99cae71b17553c0f937ea433b"
       prover_id = "0x0000000000000000000000000000000000000000000000000000000000000000"
 
         [inputs.previous_rollup.public_inputs.constants.last_archive]
-        root = "0x2d84822e545f3f6e16012e854f9327cee518281ec5f6925ae461d64328e692ee"
+        root = "0x0c2eb70ed9c405d7895991207fcf58411017ab8481fba43c23b8d01f9ee6d826"
         next_available_leaf_index = "0x0000000000000000000000000000000000000000000000000000000000000002"
 
         [inputs.previous_rollup.public_inputs.constants.l1_to_l2_tree_snapshot]
@@ -624,10 +624,10 @@ next_available_leaf_index = "0x0000000000000000000000000000000000000000000000000
   "0x18d6aae3ab4a271abd590cb98267825b70de1e9e5c339356e94ea5fc7feba64b"
 ]
         state = [
-  "0x2f7854ba50d6143cd7c2d0aab9074594e7443021752b672f532fb28036bdf464",
-  "0x2d6155d605fc85dbd2c44a9dc2d61848c1a1abe0536b44e4482c9ac6024a77d5",
-  "0x236c352e3ae68c93a62504965bc9e8778e07c0cd338fe798a2c88b9fdb4ceae7",
-  "0x032fc1b246541ca6be41be6375543d448d6b21e06b2cb16d26c42bc97deafdfc"
+  "0x1e20e5a8cee556f890f804b4c34dfa9d0c0691184ef311d07a3360c7a3ed0abb",
+  "0x2f58f461552b8e4a5eb0e2390a113ced8ed4fbe200eae42042548cc471aa8302",
+  "0x0f289d4b3c928a76f3d6b02eb37d55a0bc9f5efe83f048e9bb6087c97ecd2629",
+  "0x030aab4986f4fd9cf6184c20046212db4094a9884e02d5fd5cb39b8c3fa73119"
 ]
         cache_size = "0x0000000000000000000000000000000000000000000000000000000000000003"
         squeeze_mode = false
@@ -642,10 +642,10 @@ next_available_leaf_index = "0x0000000000000000000000000000000000000000000000000
   "0x00000000000000000000000000000000000000000000000000000000b7e5c34c"
 ]
         state = [
-  "0x2dc74882aa099ff2d05d3ee7a9f8bfdddbbdedba13a6a544b7a97b6f834bd1d6",
-  "0x2002908e197ef2363618c1ada3e9533d21bc4f30c720c5d5957a90ddbbf42748",
-  "0x1bc50bd10705cd3e35a0ddf12d2d20342247e2d9f66bd2ef3f8c87afab432a9f",
-  "0x099fa9fa9487c303774e42ca2c7897603abf9ca25584364229ef9593af312d96"
+  "0x03a9c278ef420de34c98d3ed9353652c050aa787564e29869ba130c93aeeb58c",
+  "0x04840897fb96b980247599b309da2659be7d79a529b8e120a3de7d0ce81a18c4",
+  "0x261c3adfec1a9c39f94e54687fc959511aa357d07907d7eb60b6c1ede39dc9f8",
+  "0x22aeae6b14550f3d8954f9f9098c4255a25eabe1d0e5e6c662a8917c3d1a0e00"
 ]
         cache_size = "0x0000000000000000000000000000000000000000000000000000000000000002"
         squeeze_mode = false
@@ -655,7 +655,7 @@ next_available_leaf_index = "0x0000000000000000000000000000000000000000000000000
     sibling_path = [
   "0x10b6730f1d1e9c6bf8d7c4b42b64b40d2603e3ae6ddbd464c3d8fcfb9e06e6d4",
   "0x014ffec160e37b6cb713c1a4e6e7058964dde1733e6734520ed72f72fd262919",
-  "0x196cbe2980734bc5d21b53464a1d00c06dad0febdec75822d79d6933dff40579",
+  "0x14bb1983f28c88ab603e0fe7e46b743829816df364ec385e2245072c3dad3e1e",
   "0x0787c8cc4cfb80390c27cdc17cb24ae198faad7989508690070b3cf40a2ae4fd",
   "0x134e9973b03d62389ee6021d35e61158807c7da3c3e6a052d365f53ab1ac214d",
   "0x118d25fdd2c4cc96d5af69bd85930dd49d101d463e3f5ee9f2cc9236384b5d41",
@@ -1812,6 +1812,10 @@ next_available_leaf_index = "0x0000000000000000000000000000000000000000000000000
   num_msgs = "0x0000000000000000000000000000000000000000000000000000000000000000"
   num_real_msgs = "0x0000000000000000000000000000000000000000000000000000000000000000"
 
+  [inputs.previous_l1_to_l2]
+  root = "0x18d6aae3ab4a271abd590cb98267825b70de1e9e5c339356e94ea5fc7feba64b"
+  next_available_leaf_index = "0x0000000000000000000000000000000000000000000000000000000000000400"
+
   [inputs.start_msg_sponge]
   num_absorbed = "0x0000000000000000000000000000000000000000000000000000000000000400"
 

noir-projects/fnd/noir-protocol-circuits/crates/rollup-block-root/Prover.toml

@@ -39,7 +39,7 @@ l1_to_l2_message_frontier_hint = [
 ]
 new_archive_sibling_path = [
   "0x0000000000000000000000000000000000000000000000000000000000000000",
-  "0x111f90dfe6a42b72a65046b7a1203d1249f522b498f11feda2da45ec757fed25",
+  "0x284d753011a1544ea66ccc1024bcc2aef83468522f0151d20454f31a06424e77",
   "0x14e4b977b2203b70e6ee1c2456eb7114d090fe4b907f631eecd0919fed432e7d",
   "0x30105bad22ddcc508b739b7c9ad87a561c569ff5cb0098a853c1c4ac21b7a037",
   "0x1e20ad4181460cbfdc74ca773502c59b890f184efe300ebad895956d318422da",
@@ -561,12 +561,12 @@ new_archive_sibling_path = [
     accumulated_mana_used = "0x000000000000000000000000000000000000000000000000000000000006b6c0"
 
       [inputs.previous_rollups.public_inputs.constants]
-      vk_tree_root = "0x0a178ec6fe216bfa6e2d25089ce97f9ff6b5c5701d2bce2a1fdf11e6dc351e3c"
+      vk_tree_root = "0x1d51e7bb66dda198c072b182a8e4809fc6614d677c914af2afa9c5376f16f06d"
       protocol_contracts_hash = "0x0727efc9473643b7abbe3c57df72d68e86b244b99cae71b17553c0f937ea433b"
       prover_id = "0x0000000000000000000000000000000000000000000000000000000000000000"
 
         [inputs.previous_rollups.public_inputs.constants.last_archive]
-        root = "0x09eab2d41c2ecafb0668fbada15342c670b7102c777e8fe01688205370be35dd"
+        root = "0x22fb435f1b17b06f994e96f8b9092935bb52d7ff089f9da911deb76fb11eafd0"
         next_available_leaf_index = "0x0000000000000000000000000000000000000000000000000000000000000002"
 
         [inputs.previous_rollups.public_inputs.constants.l1_to_l2_tree_snapshot]
@@ -624,10 +624,10 @@ next_available_leaf_index = "0x0000000000000000000000000000000000000000000000000
   "0x1e1c597744057b88e39a9780ed087c39b1fc42864e05ef03a59ebd9e96b70b00"
 ]
         state = [
-  "0x1340f2f29cb8306ff70bcc4ae858857833524119183778ecc6ad42461cffd2ec",
-  "0x0ded042d3f2742a39c12f49fea7578be6ca12907c913496a96aac8a4beab7da7",
-  "0x07fce0817fe38521b58a9910cc882430a755bfe4141eb684f9e13bb55ea08d59",
-  "0x2c426510a61e082128e8b428a5af495ed7351f768d421f3af48442dde4997151"
+  "0x2a5cf7199bc1a5f8871b4fea5c6f99f5dba43db93b819ca26d2fa8dbbdc1f82c",
+  "0x011cf649f43e718babd7d68caa9d36ccebf49e536e764b6674fc8ca18e8a4ada",
+  "0x17d68b99d3347b9908ca791c2a098777db49c111ac24b850a48e36fd0329b4e4",
+  "0x038347b13467b09e427b9f4154c2d36b1cd02bd1b08d8ea406687512f0b16a79"
 ]
         cache_size = "0x0000000000000000000000000000000000000000000000000000000000000002"
         squeeze_mode = false
@@ -642,10 +642,10 @@ next_available_leaf_index = "0x0000000000000000000000000000000000000000000000000
   "0x00000000000000000000000000000000000000000000000000000000b7f9d44d"
 ]
         state = [
-  "0x1f28dcf22870bc323df6d045183ab4e620f63c728f91adb34989c7f0fa15014e",
-  "0x11f9c62ca437d779c42395acdfa9aa365198c2c31c659401d9f436e898895308",
-  "0x22b47289bb74cee917c189d8d045521b296102ee2cd45d4a78df74c2d5fa0f7b",
-  "0x234bafa32c222e8d0eb62eaede11e3bfa20b4287f68b880c744abe51ac88ab6d"
+  "0x2d2cd78abcdb723be782846e1ad64ae3ab9aa2260689da6ae7475677befc11c2",
+  "0x0250e73c2089d502af086b4d31fc90a61b62f71901e93588a19421c1e54417db",
+  "0x1225c74c7caef02691c6f3a18a18ed4ce10675ec8286339594bef2cee3bcdf01",
+  "0x0b9145803dfcf3b0958502feb867fdbb62eb5726361b0971f704ffe19719c0af"
 ]
         cache_size = "0x0000000000000000000000000000000000000000000000000000000000000001"
         squeeze_mode = false
@@ -656,7 +656,7 @@ next_available_leaf_index = "0x0000000000000000000000000000000000000000000000000
   "0x1fc39d0a428c8536dbca551ea79848acf67d89d090fd8c643c7d90f2e8f32340",
   "0x12ce5a49a1ceca53ada7bee003f929bbd65abaa74e8072a81f304c2c96c44e31",
   "0x2dd71474f7775d87b6c2986ace5f654686583f0970d7400b1ccf8096dad131b5",
-  "0x0aca02f69b05a42958d30ced7da19a9e135e0c83b75e72ae5f7e2bec714a418f",
+  "0x1bd9ea476112f37bb5ce44ce3c3f0d2e62e20993253269166a016eda81ac1007",
   "0x134e9973b03d62389ee6021d35e61158807c7da3c3e6a052d365f53ab1ac214d",
   "0x118d25fdd2c4cc96d5af69bd85930dd49d101d463e3f5ee9f2cc9236384b5d41",
   "0x19dc50e83dfaeddfc1eb7b19cfcf39ef4b5608eecfaf54180697ea982b7bebbd"
@@ -1273,12 +1273,12 @@ next_available_leaf_index = "0x0000000000000000000000000000000000000000000000000
     accumulated_mana_used = "0x0000000000000000000000000000000000000000000000000000000000000000"
 
       [inputs.previous_rollups.public_inputs.constants]
-      vk_tree_root = "0x0a178ec6fe216bfa6e2d25089ce97f9ff6b5c5701d2bce2a1fdf11e6dc351e3c"
+      vk_tree_root = "0x1d51e7bb66dda198c072b182a8e4809fc6614d677c914af2afa9c5376f16f06d"
       protocol_contracts_hash = "0x0727efc9473643b7abbe3c57df72d68e86b244b99cae71b17553c0f937ea433b"
       prover_id = "0x0000000000000000000000000000000000000000000000000000000000000000"
 
         [inputs.previous_rollups.public_inputs.constants.last_archive]
-        root = "0x09eab2d41c2ecafb0668fbada15342c670b7102c777e8fe01688205370be35dd"
+        root = "0x22fb435f1b17b06f994e96f8b9092935bb52d7ff089f9da911deb76fb11eafd0"
         next_available_leaf_index = "0x0000000000000000000000000000000000000000000000000000000000000002"
 
         [inputs.previous_rollups.public_inputs.constants.l1_to_l2_tree_snapshot]
@@ -1336,10 +1336,10 @@ next_available_leaf_index = "0x0000000000000000000000000000000000000000000000000
   "0x00000000000000000000000000000000000000000000000000000000b7f9d44d"
 ]
         state = [
-  "0x1f28dcf22870bc323df6d045183ab4e620f63c728f91adb34989c7f0fa15014e",
-  "0x11f9c62ca437d779c42395acdfa9aa365198c2c31c659401d9f436e898895308",
-  "0x22b47289bb74cee917c189d8d045521b296102ee2cd45d4a78df74c2d5fa0f7b",
-  "0x234bafa32c222e8d0eb62eaede11e3bfa20b4287f68b880c744abe51ac88ab6d"
+  "0x2d2cd78abcdb723be782846e1ad64ae3ab9aa2260689da6ae7475677befc11c2",
+  "0x0250e73c2089d502af086b4d31fc90a61b62f71901e93588a19421c1e54417db",
+  "0x1225c74c7caef02691c6f3a18a18ed4ce10675ec8286339594bef2cee3bcdf01",
+  "0x0b9145803dfcf3b0958502feb867fdbb62eb5726361b0971f704ffe19719c0af"
 ]
         cache_size = "0x0000000000000000000000000000000000000000000000000000000000000001"
         squeeze_mode = false
@@ -1354,10 +1354,10 @@ next_available_leaf_index = "0x0000000000000000000000000000000000000000000000000
   "0x00000000000000000000000000000000000000000000000000000000b80de34e"
 ]
         state = [
-  "0x134946855417bcc442f889686f1d56a235ef003975c044d0247a83233b410758",
-  "0x1b72a1748313dbe8d809b3fcbafc90c42245820ec62b0dad170882743ff5229f",
-  "0x0c028e173991130ade8d45c7ca581aba2786fafd52c6204542bedbd83554d15e",
-  "0x1d53b7e7cf226e419d853dc8cfc46ecd424838a883d07356d2a1fa811e38c34e"
+  "0x081ffd41f516611101fabed22dbbba6e6bf5ac0eff43aa020fb8407fe66471be",
+  "0x07e1027927852cc3291b4d5b979823dfacab903c13b5bd3f2590388210b608ed",
+  "0x17c059bfe7dca4a328bf27482e1c7d33887c0ff69b69f2be89eb754266e263ed",
+  "0x1e85bde3c04a4baa1d2d6e1237d10ced927e777bbd7fa7c31fce28cdd79b92ca"
 ]
         cache_size = "0x0000000000000000000000000000000000000000000000000000000000000003"
         squeeze_mode = false
@@ -1367,7 +1367,7 @@ next_available_leaf_index = "0x0000000000000000000000000000000000000000000000000
     sibling_path = [
   "0x10b6730f1d1e9c6bf8d7c4b42b64b40d2603e3ae6ddbd464c3d8fcfb9e06e6d4",
   "0x014ffec160e37b6cb713c1a4e6e7058964dde1733e6734520ed72f72fd262919",
-  "0x196cbe2980734bc5d21b53464a1d00c06dad0febdec75822d79d6933dff40579",
+  "0x14bb1983f28c88ab603e0fe7e46b743829816df364ec385e2245072c3dad3e1e",
   "0x0787c8cc4cfb80390c27cdc17cb24ae198faad7989508690070b3cf40a2ae4fd",
   "0x134e9973b03d62389ee6021d35e61158807c7da3c3e6a052d365f53ab1ac214d",
   "0x118d25fdd2c4cc96d5af69bd85930dd49d101d463e3f5ee9f2cc9236384b5d41",
@@ -2524,6 +2524,10 @@ next_available_leaf_index = "0x0000000000000000000000000000000000000000000000000
   num_msgs = "0x0000000000000000000000000000000000000000000000000000000000000000"
   num_real_msgs = "0x0000000000000000000000000000000000000000000000000000000000000000"
 
+  [inputs.previous_l1_to_l2]
+  root = "0x18d6aae3ab4a271abd590cb98267825b70de1e9e5c339356e94ea5fc7feba64b"
+  next_available_leaf_index = "0x0000000000000000000000000000000000000000000000000000000000000400"
+
   [inputs.start_msg_sponge]
   num_absorbed = "0x0000000000000000000000000000000000000000000000000000000000000400"
 

noir-projects/fnd/noir-protocol-circuits/crates/rollup-lib/src/abis/mod.nr

@@ -1,6 +1,6 @@
 // TODO: Move to types/src/abis/rollup/
-// `l1_to_l2_message_sponge` is declared first because the serde `#[derive(...)]` macro requires a field type's module
-// to be declared before any struct that embeds it (see the ordering note in `types/src/lib.nr`).
+// `l1_to_l2_message_sponge` is declared first because the `#[derive(Serialize, Deserialize)]` macros require a field
+// type's module to be declared before any struct that embeds it (see the ordering note in `types/src/lib.nr`).
 mod l1_to_l2_message_sponge;
 mod l1_to_l2_message_bundle;
 mod public_chonk_verifier_public_inputs;

noir-projects/fnd/noir-protocol-circuits/crates/rollup-lib/src/abis/root_rollup_public_inputs.nr

@@ -18,8 +18,7 @@ pub struct RootRollupPublicInputs {
     // The out hash of the first checkpoint in the epoch is inserted at index 0, the second at index 1, and so on.
     pub out_hash: Field,
     // Inbox rolling-hash chain segment consumed across the epoch: the chain value before the first checkpoint's
-    // messages and after the last checkpoint's. Continuity within the range is enforced by the checkpoint merges;
-    // L1 passes both through to proof verification without validating them until the Fast Inbox flip.
+    // messages and after the last checkpoint's. Continuity within the range is enforced by the checkpoint merges.
     pub previous_inbox_rolling_hash: Field,
     pub end_inbox_rolling_hash: Field,
     // Checkpoint header hashes for each checkpoint in this epoch.

noir-projects/fnd/noir-protocol-circuits/crates/rollup-lib/src/block_merge/utils/validate_consecutive_block_rollups.nr

@@ -42,8 +42,6 @@ fn assert_prev_block_rollups_follow_on_from_each_other(
         "Mismatched timestamps: expected right.timestamp to match left.timestamp",
     );
 
-    // TODO: Consider extracting into its own function:
-
     // `is_first_block` originates from the first block root only, and is propagated through all merge steps via the
     // left rollup only (see merge_block_rollups.nr). We prevent the right rollup from carrying it, so it's impossible
     // for any block but the first to propagate `is_first_block == true` up to the checkpoint root.

noir-projects/fnd/noir-protocol-circuits/crates/rollup-lib/src/block_root/block_root_rollup.nr

@@ -1,8 +1,12 @@
 use crate::{
     abis::{BlockRollupPublicInputs, L1ToL2MessageBundle, L1ToL2MessageSponge, TxRollupPublicInputs},
-    block_root::components::{BlockRollupPublicInputsComposer, validate_previous_rollups},
+    block_root::components::{
+        BlockRollupPublicInputsComposer, validate_l1_to_l2_tree_snapshot_in_constants,
+        validate_previous_rollups,
+    },
 };
 use types::{
+    abis::append_only_tree_snapshot::AppendOnlyTreeSnapshot,
     constants::{
         ARCHIVE_HEIGHT, L1_TO_L2_MSG_TREE_HEIGHT, PRIVATE_TX_BASE_ROLLUP_VK_INDEX,
         PUBLIC_TX_BASE_ROLLUP_VK_INDEX, TX_MERGE_ROLLUP_VK_INDEX,
@@ -17,11 +21,15 @@ pub struct BlockRootRollupPrivateInputs {
     pub(crate) previous_rollups: [RollupHonkProofData<TxRollupPublicInputs>; 2],
     // L1-to-L2 message bundle inserted by this block.
     pub(crate) message_bundle: L1ToL2MessageBundle,
+    // The l1-to-l2 tree snapshot this block builds on (the previous block's post-insertion snapshot). It is a witnessed
+    // value, pinned by block-merge continuity (`right.start_state == left.end_state`) to the previous block's end
+    // state; since the checkpoint root forces the leftmost block to be a first-block variant, every non-first block
+    // has a left neighbour that pins it.
+    pub(crate) previous_l1_to_l2: AppendOnlyTreeSnapshot,
     // Message sponge inherited from the previous block. Checked against the previous block's `end_msg_sponge` in the
     // block merge or checkpoint root circuit.
     pub(crate) start_msg_sponge: L1ToL2MessageSponge,
-    // Frontier hint for appending the bundle to the l1-to-l2 message tree (validated against the tree snapshot in the
-    // constants, which is where the l1-to-l2 tree stands for this non-first block).
+    // Frontier hint for appending the bundle to the l1-to-l2 message tree (validated against `previous_l1_to_l2`).
     pub(crate) l1_to_l2_message_frontier_hint: [Field; L1_TO_L2_MSG_TREE_HEIGHT],
     // Hint for inserting the new block hash to the last archive.
     pub(crate) new_archive_sibling_path: [Field; ARCHIVE_HEIGHT],
@@ -43,18 +51,29 @@ pub struct BlockRootRollupPrivateInputs {
 ///
 /// VkIndex: BLOCK_ROOT_ROLLUP_VK_INDEX
 pub fn execute(inputs: BlockRootRollupPrivateInputs) -> BlockRollupPublicInputs {
-    validate_previous_rollups(inputs.previous_rollups, ALLOWED_PREVIOUS_VK_INDICES);
-
     let previous_rollups = inputs.previous_rollups.map(|rollup| rollup.public_inputs);
-    // Non-first blocks build on the l1-to-l2 tree snapshot carried in the checkpoint constants.
-    let previous_l1_to_l2 = previous_rollups[0].constants.l1_to_l2_tree_snapshot;
-    BlockRollupPublicInputsComposer::new_from_two_rollups(previous_rollups)
+    // Non-first blocks build on a witnessed start snapshot (pinned by block-merge continuity to the previous block's
+    // end state) rather than the constants value, so this block can append its own bundle and still assert the tx
+    // constants carry the post-bundle snapshot.
+    let mut composer = BlockRollupPublicInputsComposer::new_from_two_rollups(previous_rollups);
+    let new_l1_to_l2 = composer
         .with_message_bundle(
             false,
-            previous_l1_to_l2,
+            inputs.previous_l1_to_l2,
             inputs.start_msg_sponge,
             inputs.message_bundle,
             inputs.l1_to_l2_message_frontier_hint,
         )
-        .finish(inputs.new_archive_sibling_path)
+        .get_new_l1_to_l2();
+
+    validate_previous_rollups(inputs.previous_rollups, ALLOWED_PREVIOUS_VK_INDICES);
+
+    // The tx constants pin the l1-to-l2 snapshot the AVM validates message reads against; asserting it equals this
+    // block's post-bundle root lets this block's txs read the messages this block inserts (same-block consumption).
+    validate_l1_to_l2_tree_snapshot_in_constants(
+        inputs.previous_rollups[0].public_inputs.constants,
+        new_l1_to_l2,
+    );
+
+    composer.finish(inputs.new_archive_sibling_path)
 }