fix: prevent building on orphan proposed blocks

[spalladino]

Motivation

Under proposer pipelining a node can receive and reexecute the block-only proposals for a checkpoint before (or without ever) receiving the enclosing proposed checkpoint. This leaves the local tip one checkpoint ahead of the checkpointed tip with no proposed checkpoint backing it. A sequencer that then builds the next checkpoint on top of that orphan tip forks the chain off a parent no other node can follow, which was the root cause behind the sentinel CI flake.

Approach

Two complementary defenses. The sequencer's checkSync refuses to proceed when the synced block's checkpoint is ahead of the checkpointed tip and no matching proposed checkpoint exists, holding the line during the window before cleanup. The archiver adds a wall-clock orphan prune that, shortly after a block's build slot ends, removes a block-only tip whose checkpoint was never proposed, restoring liveness even while L1 is quiet.

Changes

• sequencer-client: checkSync rejects syncing onto a proposed block with no matching proposed-checkpoint tip/data, logging a descriptive warning.
• archiver: new pruneOrphanProposedBlocks on the L1 synchronizer, run from Archiver.sync() after the inbound queue drains and before L1 sync; prunes after start(blockSlot) + grace using the epoch-cache pipelining offset and emits L2PruneUncheckpointed. The existing L1-sync prune is preserved (shared prune/emit helper).
• archiver/stdlib/foundation config: new orphanProposedBlockPruneGraceSeconds in ArchiverSpecificConfig, archiver config mappings (ARCHIVER_ORPHAN_PROPOSED_BLOCK_PRUNE_GRACE_SECONDS), mapArchiverConfig, the synchronizer/archiver config types, and a new EnvVar.
• aztec-node: defaults the grace window from blockDurationMs / 1000 when unset, falling back to MIN_EXECUTION_TIME; the archiver factory also defaults to MIN_EXECUTION_TIME.
• sequencer-client (tests): orphan tip returns undefined and warns; matching proposed checkpoint proceeds.
• archiver (tests): no prune before grace; prune + event after grace; no prune when a matching proposed checkpoint exists; queued proposed checkpoint is processed before the prune.

[AztecBot]

Flakey Tests

🤖 says: This CI run detected 1 tests that failed, but were tolerated due to a .test_patterns.yml entry.

\033FLAKED\033 (8;;http://ci.aztec-labs.com/bf10bc4debb0b908�bf10bc4debb0b9088;;�):  yarn-project/end-to-end/scripts/run_test.sh simple src/e2e_p2p/sentinel_status_slash.parallel.test.ts "slashes the proposer with INACTIVITY when checkpoint validation records unvalidated" (215s) (code: 0) group:e2e-p2p-epoch-flakes