Immutable catalog generation implementation plan

Issue: #49

Outcome

Catalog discovery and DataFusion listing pin one immutable Arc containing both table names and schemas. Refresh atomically replaces the Arc, eliminating mixed-generation responses and full-listing clones.

Steps

1. Replace mutable snapshot contents with RwLock<Arc<CatalogGeneration>> and expose read-only generation accessors.
2. Publish a newly built generation only after successful complete refresh.
3. Route Query/Flight schema and table discovery through one pinned Arc.
4. Add deterministic pointer-identity, refresh-isolation, failure, and Flight response tests.
5. Run strict clippy, spec lifecycle, full gate, review, and verification.

Safety properties

• Old generations remain valid for in-flight readers until their Arc drops.
• Names and schemas are immutable and share one publication boundary.
• Failed refresh cannot mutate or replace the last-good allocation.
• Pointer cloning is O(1); iteration cost follows the requested response.