Make speaker profile merges atomic on current main

[r3dbars]

Summary

Fresh current-main replacement for #1546/#1566/#1567 after #1564 advanced main.

• keeps current speaker finalization and transcript-owner behavior
• makes profile merge and transcript rewrite rollback-safe
• propagates persistence failures from the naming mutation batch
• preserves the existing speaker side-effect coordinator and tests

Proof

• source-list validation and git diff --check
• 116 focused speaker tests on the reviewed predecessor
• Claude advisory review found no P1 blocker
• fresh current-main hosted proof is required before merge

Manual boundary

Settings speaker merge remains manual QA and is not claimed.

Sole serialized merger: Codex final reviewer/merger; Claude advisory risk review; Local unavailable; Windows skipped.