Mirrors the pattern used by headroom.ccr_backend so memory store,
vector index, and text index backends can be registered via setuptools
entry points.
- EXTERNAL enum value on StoreBackend, VectorBackend, TextBackend
- Optional *_backend_name fields on MemoryConfig
- entry_points(group=...) lookup in _create_{store,vector_index,text_index}
- New test_factory_external.py (7 tests) covering load / missing-name /
unknown-name paths
Default behavior (SQLITE + AUTO + FTS5) unchanged.
Extension groups:
headroom.memory_store
headroom.memory_vector
headroom.memory_text
