Phase 1 of the file-based storage plan. Pure library, no runtime
behavior change yet (editors still use SQLite).
- BadnoteSidecar: per-file annotation document (schema-versioned)
holding per-page ink (EditorStroke JSON), text highlights,
scratch-link anchors (ScratchLink JSON) each with its own
scratchpad (InkStroke world-coord JSON), and bookmarks. Reuses the
existing toJson formats — no parallel stroke format.
- SidecarStore.writeAtomic: temp-file + rename atomic write keeping a
.bak; read() falls back to .bak on a missing/corrupt primary.
Round-trip + atomic-write + .bak-recovery tests. analyze clean,
322 tests green.
