backport: assumeutxo M5 — loadtxoutset end-to-end (bitcoin#27596 + cluster + E2E)

[PastaPastaPasta]

Issue being fixed or feature implemented

M1–M4 (#50, #51, #52, #53) built the persistence, role, and evo
snapshot-format machinery; this milestone exposes it end to end via
loadtxoutset. This is milestone 5/7, the largest unit of the whole series
(43 files in the core RPC unit alone). The execution plan and per-unit ledger
(M5 section) are tracked outside the repo in a private gist, not checked in:
https://gist.github.com/PastaPastaPasta/aa52b1f89fb74a0566ba3b5e15afab6a

PR 5/7 in the stacked series — base is M4 (assumeutxo/m4-evo-snapshot).

What was done?

• Partially backported the first commit of bitcoin#25494 to introduce
  BlockInfo, MakeBlockInfo, and kernel/chain.{h,cpp}, followed by the
  ChainstateRole portion of bitcoin#27596. These are the direct
  prerequisites for the role-tagged notification work below.
• bitcoin#27596 (22 upstream commits) — loadtxoutset and getchainstates
  RPCs; AssumeutxoData modernization preserving evo_hash; role-tagged
  notifications replace the M2 BlockConnected/ChainStateFlushed
  suppression guards (17-subscriber audit table in review docs; Dash-only
  MN-list/UI notifications stay active-gated; UpdatedBlockTip/
  BlockDisconnected stay active-only per upstream); net_processing
  background-chain download; blockfile segmentation + multi-chainstate
  pruning; mempool swap on activation; feature_assumeutxo.py ported.
  Round 1 review found two issues: a height-200 regtest entry retaining a
  null blockhash (caused a LoadBlockIndex assert abort on restart) and an
  omitted upstream test hunk — both fixed with deterministic fixture hashes
  and restart coverage.
• Follow-up cluster bitcoin#28562..#28838 (13 PRs). Gate1 raised an
  objection confirmed by gate2 review: the initial Dash adaptation had
  globally weakened CheckBlockIndex's exact parent-sum nChainTx assert to
  paper over off-by-one regtest AssumeutxoData values — rejected as a
  workaround; fixed by restoring the upstream assert byte-identical and
  correcting the params instead. #28685 regenerated regtest
  hash_serialized_3 vectors (evo_hash independently verified unaffected);
  #28698 routes an upstream Assert through Dash's AbortNode fatal path;
  wallet_assumeutxo.py ported; #28852 skipped as superseded by #29553
  in M6.
• E2E capstone: feature_assumeutxo_dash.py plus regtest -assumeutxodata
  authorization. Full lifecycle across 8 MNs/EvoNodes past DIP3/DIP8/DIP0024/
  v19/v20, rotated + non-rotated quorums, dump→loadtxoutset→deep-state
  verify, ChainLock/InstantSend verification pre-completion, signing/MN-mode
  refusals, background completion with evo comparisons, 4-phase restarts
  (exercising M3's crash recovery), and UTXO-only/tampered-evo rejection.
  Round 1 review found the equality checks compared identifiers rather than
  deterministic state — fixed with normalized ProTx + per-quorum
  member/key/commitment comparison. New regtest-only
  -assumeutxodata=<h>:<utxo_hash>:<evo_hash>:<nchaintx>:<blockhash> (needed
  because MN fixtures aren't deterministic across runs); ignored on
  non-regtest, duplicates/collisions rejected. Four production fixes were
  flushed out by the full lifecycle run.
• Two orchestrator-found fixes: per-chainstate prune target vs. Dash's
  -prune=550 (upstream clamped targets to MIN_DISK_SPACE_FOR_BLOCK_FILES,
  breaking supported sub-minimum targets — fixed while keeping the
  multi-chainstate division); interfaces::BlockInfo moved to
  kernel/chain.h to break a circular dependency.

Runtime activation is supported for ordinary nodes; loadtxoutset REFUSES
masternode mode (live signing contexts aren't rebindable yet — documented
follow-up for a later series, not this one).

How Has This Been Tested?

• make check: exit 0, 0 failures (final tree).
• Functional battery (16): assumeutxo trio (feature_assumeutxo,
  feature_assumeutxo_dash E2E ~128s, wallet_assumeutxo),
  rpc_dumptxoutset, dip3 ×2, llmq signing ×2/rotation/chainlocks, mnehf,
  asset_locks, init, reindex, pruning, txindex — all passed.
• Lint: circular-dependencies and whitespace PASS; python lint deferred to CI.
• 2 rounds for assumeutxo (2) bitcoin/bitcoin#27596, 2 for the 13-PR cluster, 2 for the E2E capstone;
  milestone gate PASS on 2026-07-11.

Breaking Changes

loadtxoutset and getchainstates are now reachable RPCs (previously
unexposed on Dash). No prior Dash release exposed these, so no compatibility
break for existing users. dumptxoutset output now includes the v3 evo
section from M4; snapshots taken before this series are not loadable (assumeutxo
is unreleased on Dash).

Checklist:

• I have performed a self-review of my own code
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation (docs rewrite lands in M7/B8)
• I have assigned this pull request to a milestone

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