Require Engine confirmation before finalizing sequenced blocks

[vmarkushin]

The sequencer previously updated forkchoice before reconstructing and checking the payload, then could return a block even when the Engine did not confirm it. That could let the orchestrator persist message consumption or sign and announce an unconfirmed block.

This change validates the reconstructed block first and requires a VALID forkchoice response before returning it. Rejection or transport failure preserves the local head. Finalization failures emit the existing terminal skipped event so a remote-source builder can continue instead of waiting indefinitely.

Extracted from #45, independently based on main after #47. This PR does not change configuration or introduce the broader cancellation/settlement redesign.

Validation:

• 25 orchestrator unit tests and 4 sequencer unit tests passed.
• Regression coverage includes hash mismatch before forkchoice, INVALID/SYNCING/ACCEPTED, transport failure, successful head commitment, and the terminal event on rejection.
• Fresh GPT-6 Astra review at high effort: approved, no actionable findings.

Remaining build-start, cancellation, and post-head-commit failure handling is tracked in the later split branches.

[codspeed-hq]

Merging this PR will not alter performance

✅ 2 untouched benchmarks

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_{Comparing codex/issue-38-02-sequencer-fcu (4feb3f6) with main (8377dc4)}