execution/execmodule: reserve execution during startup

[yperbasis]

Closes #22925.

Problem

The execution semaphore was available as soon as NewExecModule returned. Engine and embedded RPC servers can start before the lifecycle calls ExecModule.Start, so a stateful engine request could run first. Payload validation could attach StateCache to a SharedDomains and start read-ahead from the pre-catch-up snapshot.

Frozen-block startup then advances durable state through cacheless SharedDomains. A pre-start reader could therefore leave or refill stale cache entries after catch-up.

End state

NewExecModule now reserves the existing execution permit before the module is exposed. ExecModule.Start owns that reservation through ProcessFrozenBlocks and releases it when startup returns. Try-acquire operations report Busy, while blocking operations wait.

Paths that do not run frozen-block initial sync explicitly finish startup. This keeps the test harness and legacy sync behavior unchanged.

No cache-populating execution path can now begin before or during frozen-block processing. ProcessFrozenBlocks remains cacheless, with no catch-up write-set copying, cache publication, or per-batch cache work.

Tests

The readiness regression test was written first and failed on main because a newly constructed module reported ready. Coverage also verifies that payload validation returns Busy before startup.

Validation on the final tree:

• go test ./execution/execmodule/... -count=1
• go test -race ./execution/execmodule/... -count=1 -timeout=10m
• make test-short
• make erigon
• make integration

make lint was run twice on the final tree. Both runs reported no issue in the changed code and stopped only on the existing macOS unused-field report for residencyOnce in db/seg/decompress.go:199, which this PR does not modify.

[Copilot]

Pull request overview

Reserves execution capacity during startup to prevent pre-catch-up state-cache access.

Changes:

• Holds the execution semaphore until startup completes.
• Releases the reservation for legacy and test paths without initial sync.
• Adds readiness and pre-start validation regression tests.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
node/eth/backend.go	Completes startup reservation across lifecycle paths.
execution/execmodule/execmoduletester/exec_module_tester.go	Preserves test harness behavior.
execution/execmodule/exec_module.go	Implements startup semaphore ownership.
execution/execmodule/exec_module_internal_test.go	Tests readiness and pre-start rejection.

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