Property tests for the core invariants

[samkeen]

Golden-vault scenario tests exist; property coverage over generated vaults is the gap. Pin the load-bearing invariants:

• Markdown round-trip (parse → serialize is lossless).
• full reindex ≡ incremental (now real and worth pinning).
• rename-keeps-every-backlink-resolving (b2 mv, user-stories.md Story 1).

From planning/tasks.md Backlog.

[samkeen]

Pointer from the #31 fix (PR #51): the full reindex ≡ incremental invariant now has a reusable comparison harness — Observable / observable_state in crates/b2-core/tests/project_embed.rs compares two indexes on exactly the invariant-relevant surface (note count, (note, seq) → chunk text, text → vector bytes, full edge rows — deliberately never rowids). A property test can generate a random vault plus a random sequence of out-of-band mutations (create / edit / delete / rename / path swap), reindex incrementally after each, and assert observable_state(incremental) == observable_state(fresh rebuild). The deletion and path-takeover cases are pinned example-wise today (reindex_prunes_a_deleted_note_like_a_full_rebuild, reindex_reconciles_a_path_taken_over_by_another_note); generation would cover their composition space. The helper lives in a test file, so lifting it into tests/common/ is the first step.

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