Replace bounded shared project notes without losing concurrent edits

Requires project write access. Read context first and pass notes.version as expected_version (zero for a new document). Maximum 250 lines and 16384 UTF-8 bytes; line endings normalize to LF. Empty Markdown clears notes. Never store secrets, logs or signed access URLs. Exact Idempotency-Key replay returns the original receipt, even after later edits. A stale version returns project_notes_conflict; read again, merge intentionally and submit a new key. Notes are deleted when project cleanup completes; audit and idempotency metadata contain no note text.