Make speaker profile merges atomic on current main

[r3dbars]

Current-main replacement for #1546 after #1563, #1538, #1549, and #1565 advanced main.

This keeps the later speaker-finalization and transcript-owner behavior from main, makes profile merge plus transcript rewrite rollback-safe, and propagates persistence failures from the naming mutation batch. It intentionally does not restore the deleted Tests/FastTests.manifest.

Proof plan:

• build-deps.sh --force
• signed local build
• fast/full Swift tests
• integration smoke
• exact-head merge simulation and independent review

Manual Settings speaker merge proof remains separate and is not claimed here.

[r3dbars]

Disposition: superseded by replacement #1571, which carries the repaired atomic speaker merge implementation on current main and has merged as e1f7440. This stale replacement is not being merged.