PBZ-PLAN: item 13 has now caught two reviewers on real hardware

Both were exercising Chunk 30 and neither expected save to land on an
existing id derived from the file basename. Upgrading the note from
"deserves a warning" to evidence that it is a usability defect.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PZnUXRsLLnt2A7pR3X4Luj