refactor(consensus): Move Forkchoice Finalize Off Queue

[refcell]

Summary

Move delegated forkchoice and finalized-head updates onto direct Engine methods. The engine processor now executes those paths directly and maps errors through the existing engine task severity handling. The old task wrappers remain as compatibility shells so the queue machinery can be deleted in the next stage without mixing behavior changes into this PR.

[vercel]

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
base	Ignored		May 5, 2026 5:23pm

[github-actions]

Review Summary

Clean refactoring that moves delegated forkchoice and finalize logic from the task queue path into direct Engine methods, while keeping the old task wrappers as thin compatibility shells.

Verified:

• Retry semantics preserved: both delegated_forkchoice() and finalize() retry Temporary errors internally with yield_now(), matching the existing EngineTask::execute() retry loop. Non-temporary errors propagate to handle_engine_task_error() as before.
• No double-retry: the _with_state methods called by both the task wrappers and the new direct methods do not contain retry loops themselves.
• Error type hierarchy is sound: DelegatedForkchoiceTaskError has From<ConsolidateTaskError> and From<FinalizeTaskError>; FinalizeTaskError has From<SynchronizeTaskError>. Severity delegation is correct.
• State broadcasting: state_sender.send_replace() only fires on success in the new direct methods, consistent with the existing pattern for build(), consolidate(), and insert_payload_with_retry().
• Idempotency on retry: if consolidate_with_state succeeds but finalize_with_state fails with a Temporary error inside delegated_forkchoice, the retry re-runs consolidation on already-updated state — this is safe because consolidation is idempotent (same behavior as the old task-based path).
• Safe head notifications: happen on the next loop iteration via drain() → send_derivation_actor_safe_head_if_updated(), same as all other already-migrated direct methods.

No issues found.