Risk: forced finalize can stall and leave callbacks registered

[acgxv]

Problem
Forced finalize could stall if the engine never emits segment_end or endpoint_reset, leaving callbacks registered indefinitely.

Impact

• Callback registrations can remain active longer than intended.
• Finalize completion is ambiguous when no update arrives.
• Harder lifecycle debugging if the engine misbehaves.

Expected
Finalize should complete (or fail fast) and ensure callbacks are cleaned up even if the engine fails to emit updates.

Notes
Repro would require a forced finalize path where the engine does not emit segment_end or endpoint_reset.