Formalizes the BaseCollection/BaseBackend contract introduced as a seam
in #413 into an interchangeability spec that third-party backends can
build to. Driven by six in-flight backend PRs (#574, #643, #665, #697,
#700, #381) each implementing the interface differently.

Key decisions captured: entry-point distribution, typed QueryResult/
GetResult replacing Chroma dict shape, daemon-first multi-palace model
via PalaceRef, required where-clause subset (incl. $contains),
mandatory embedder injection with model-identity validation, capability
tokens, shared pytest conformance suite, and a backend-neutral
migrate/verify CLI.

…nd registry (RFC 001 §10)

Advances RFC 001 §10 cleanup so backend-author PRs (#574 LanceDB, #665 Postgres,
#700 Qdrant, #697 hosted, #643 PalaceStore, #381 Qdrant) have a stable target
to align against.

Scope (this PR):

- Typed QueryResult / GetResult dataclasses replace Chroma's dict shape at
  the BaseCollection boundary (§1.3). A transitional _DictCompatMixin keeps
  existing callers working while the attribute-access migration proceeds.
- BaseCollection is now kwargs-only across add/upsert/query/get/delete/update
  with ABC defaults for estimated_count/close/health and a non-atomic default
  update() (§1.1–1.2).
- PalaceRef replaces raw path strings at the backend boundary (§2.2).
- BaseBackend ABC with get_collection/close_palace/close/health/detect (§2.3).
- mempalace.backends entry-point group + in-tree registry with
  resolve_backend_for_palace priority order matching §3.2–3.3.
- ChromaCollection normalizes chroma returns into typed results; unknown
  where-clause operators raise UnsupportedFilterError (no silent drop, §1.4).
- ChromaBackend absorbs the inode/mtime client-cache freshness check
  previously duplicated in mcp_server._get_client() (§10 + PR #757).
- searcher.py migrated to typed-attribute access as the reference call
  site; remaining callers land in a follow-up.
- pyproject: chroma registered via [project.entry-points."mempalace.backends"].

Out of scope (explicit follow-ups):

- Full caller migration off the dict-compat shim across palace.py,
  mcp_server.py, miner.py, convo_miner.py, dedup.py, repair.py, exporter.py,
  palace_graph.py, cli.py, closet_llm.py.
- Embedder injection + three-state EmbedderIdentityMismatchError check (§1.5).
- maintenance_state() / run_maintenance() benchmark hooks (§7.3).
- AbstractBackendContractSuite full coverage (§7.1–7.2).
- mempalace migrate / mempalace verify CLI rewrites through BaseCollection (§8).

Tests: 970 passed (up from 967 on develop); new coverage for typed results,
empty-result outer-shape preservation, \$regex rejection, registry lookup,
priority resolver, and PalaceRef-kwarg ChromaBackend.get_collection.

Refs: #743 (RFC 001), #989 (RFC 002 tracking issue).