Verify Beacon state across process restarts

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Why

In-memory load tests cannot prove that graceful shutdown writes a usable checkpoint or that incompatible startup leaves it untouched. Exercise the real CLI across SIGTERM and restart.

What

• Compare retained blocks, receipts, EVM block hashes, and exact JSON/SSZ blob bytes.
• Check canonical clock and simulated-finality behavior after restart.
• Reject incompatible startup without rewriting the saved state file.
• Bound child-process waits and clean up children on failure.

Notes

• Process-signal tests run on Unix; state and routing regressions remain cross-platform.
• Implementation, tests, and review assisted by Amp.

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Local qualification at f514786ab: 347/347 selected offline tests passed across anvil, anvil-core, anvil-rpc and anvil-server with zero retries, including real SIGTERM/restart and incompatible-startup preservation. All-target/all-feature Anvil Clippy, workspace formatting and the no-default-features build passed. Independent reviews and follow-up reviews have no unresolved findings.

Review map: independent fixes #77, #78, #79; feature chain #80 → #81 → #82 → #83 → #84 → #85 → #86 → #87 → #88. #84 includes the #77/#78 branches, and #85 includes #79. Each feature PR targets its immediate parent.

CI qualification as of October 5, 21:25 UTC: #86 and #87 passed 341 and 345 Anvil tests respectively, with the same five Forge failures seen on the original PRs. Workspace crate checks passed on #86 and #88. The latest five runs had 29 cancellations without acquiring a runner, and #87 Clippy lost its runner during dependency compilation. The unchanged rustls advisory RUSTSEC-2026-0285 also fails cargo-deny. Earlier runs show pre-existing external-fixture dependency/RPC failures; the latest #87 external-fixture job is still running. All other jobs in the latest five runs have finished. Cancelled checks are unverified, not passes.

These drafts are not merge-ready. Dependency/CI maintenance needs a separate scope decision; no checks were disabled or manually rerun. The original #75/#76 branches and immutable devnet pin remain unchanged. This qualification does not claim live Base snapshot or Denim acceptance.

Comment generated by Amp.

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CI repairs from #89 are included at e04db3462. The selected Anvil suite passed 347/347 locally with zero retries, including both process-restart tests; the hosted main test suite passed too. The feature diff against the parent is unchanged.

This PR is not fully green: the external Convex fixture hit public archive-RPC rate limits, then timed out. Its other eleven external fixtures passed. Reliable external verification needs a CI-approved archive endpoint in the existing HTTP_ARCHIVE_URLS repository secret. A cancelled duplicate workflow is also being rerun to clear its stale aggregate result. No checks were disabled.

The earlier feature review evidence is stack-level, not twelve independently recorded per-parent review cycles. The maintenance changes received their own independent reviews. Comment generated by Amp.