When PersonService renames a person while a transcriber is editing a
block that mentions them, the block-save endpoint returns 409 (carrying
the new ErrorCode.PERSON_RENAME_CONFLICT from PR-A). saveBlock now:
1. Refetches the latest server snapshot of the block.
2. Calls mergeBlockOnConflict to combine: server's mentionedPersons
(post-rename displayNames win) + transcriber's unsaved text + any
local-only mentions added since the last save.
3. Updates the local block state with the merged result.
4. Re-throws so the autosave indicator surfaces the conflict and the
pending payload is preserved for retry (B12).
The merge logic is a pure function so it can be unit-tested in
isolation and reused for any future conflict-resolution scenarios.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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