Provider generation cache implementation plan

Issue: #45

Outcome

Repeated SQL planning in one query replica reuses a bounded, immutable TableProvider for the exact registry-visible table generation. Concurrent misses perform one storage open/provider build, while append and recreate select fresh providers.

Steps

1. Add a typed provider-generation key and bounded async cache to the shared catalog state.
2. Resolve providers through one fallible singleflight initializer; never cache storage failures or missing tables.
3. Add deterministic fake-engine tests for concurrency, version and incarnation separation, failure retry, and capacity.
4. Document the query-replica cache boundary and generation identity.
5. Run strict clippy, spec lifecycle, full gate, and independent review and verification.

Safety properties

• Providers remain pinned to the registry version used in their key.
• Location and incarnation prevent reuse across physical replacement and legacy/recreated registration transitions.
• Cache eviction changes performance only; immutable providers remain safe for in-flight readers holding an Arc.
• Initializer errors and missing engine handles do not poison future reads.
• A per-table local registration epoch makes write invalidation linearizable against earlier in-flight cache fills.