Add SqliteVecBackend / SqliteVecCollection as a fully working,
zero-dependency alternative to ChromaDB. All core backend protocol
requirements (RFC 001) are met; 104 new tests cover every code path.

Key changes
-----------
mempalace/backends/sqlite_vec.py (new)
  - Multi-collection: each collection_name maps to its own SQL table
    inside palace.db; _safe_table_name() prevents SQL injection.
  - Dynamic embedding dimension: vec virtual table is created lazily on
    the first write that contains an embedding; the actual dimension is
    detected from len(embedding) and stored.  DimensionMismatchError is
    raised if a subsequent write arrives with a different dimension.
    On reconnect the dimension is read back from sqlite_master so the
    vec table is never recreated with the wrong dim.
  - ANN over-fetch + post-filter: when sqlite-vec is available the query
    fetches n_results * _ANN_OVERFETCH (10x) ANN candidates, applies
    Python-side where / where_document filters, and returns the top
    n_results survivors.  If survivors < n_results the query falls back
    to a full brute-force cosine scan so correctness is always guaranteed.
    When sqlite-vec is absent the brute-force path is used directly.

mempalace/backends/registry.py
  - _register_builtins() registers SqliteVecBackend under "sqlite_vec"
    so get_backend("sqlite_vec") and resolve_backend_for_palace() work
    out of the box.

mempalace/backends/__init__.py
  - SqliteVecBackend and SqliteVecCollection added to public surface and __all__.

tests/test_sqlite_vec.py (new, 104 tests / 2 skipped without sqlite-vec)
  - Section 1: pure-Python utility functions (_pack_f32, _unpack_f32,
    _cosine_distance, _cosine_brute).
  - Section 2: _meta_matches -- all operators ($eq, $ne, $in, $nin, $gt,
    $gte, $lt, $lte, $contains, $and, $or, nested).
  - Section 3: SqliteVecCollection CRUD (add, upsert, update, delete, get,
    count), LIMIT/OFFSET, close/health.
  - Section 4: query() brute-force path (no sqlite-vec needed, CI-safe).
  - Section 5: SqliteVecBackend lifecycle (caching, close, health).
  - Section 6: detect() classmethod.
  - Section 7: registry integration.
  - Section 8: end-to-end integration (round-trips, concurrency, persistence).
  - Section 9: multi-collection isolation and _safe_table_name validation.
  - Section 10: dynamic dimension detection and ANN over-fetch behaviour.