feat: git-like project versioning (v1)

Linear per-project history per notes/project-versioning.md: named
checkpoints (thumbnail via renderer.captureFrame) plus auto-checkpoints
on save (60s throttle, ring of 20, named never rotate). Restore writes a
safety checkpoint first, then swaps the document outside undo/redo.
Media assets referenced by any version are pinned against deletion.
Migration v31-v32 stamps the schema; the version store lives in its own
IDB database with an in-memory fallback for tests. Versions popover in
the editor header for jump/rename/delete. 12 new tests.

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