Add object cache for ID lookups

[colemancda]

Summary

Adds an in-memory object cache to SQLiteDatabase, mirroring CoreData's row cache. Repeated fetch(_:for:) calls return the cached ModelData without touching SQLite, skipping both the row query and the per-relationship queries that derive to-many values. Predicate fetches (fetch(_ fetchRequest:)) also register their results, so ID lookups following a list fetch are free.

Design

• The cache is actor-isolated state ([EntityName: [ObjectID: ModelData]]), so no additional synchronization is needed.
• Invalidation: a write to entity E touches E's own table, the foreign key columns of E's relationship destinations, and their shared join tables. Derived to-many values are computed only from those same foreign keys and join tables, so every write (insert/delete) drops the cached objects of E plus E's destination entities. Whole entity caches are dropped rather than single objects because a write can affect sibling rows of the same entity — e.g. reassigning a person to a new team also changes the old team's derived members.
• The cache assumes the SQLiteDatabase actor is the database's only writer — the same coherency contract as separate CoreData stacks sharing one store file. SQLiteViewContext reads its own connection directly and always observes committed state.

Benchmarks

Measured against a production-scale model (13 entities, ~11K objects, file-backed database) with 100 single-object fetches by ID, two passes over the same IDs (M-series Mac):

Pass	Before	After
Cold (first fetch per ID)	~0.04–0.12s	unchanged
Warm (repeat fetch per ID)	same as cold	~0.001–0.002s

Warm ID lookups — the shape detail screens hit when reopening records — go from re-querying every time to effectively free, and are competitive with CoreData's row cache for the equivalent workload.

Tests

CacheTests covers cache population (by-ID and via predicate fetch registration) and every invalidation path: direct updates, inverse to-many changes from the foreign-key side, reassignment side effects on sibling rows, deletes, and delete-nullification of cached references. Full suite passes (50 tests).