Catalog directory generation implementation plan

Goal

Replace every Query replica’s steady five-second full registry scan with one strongly consistent generation point read, without weakening mixed-version rollout safety or immutable discovery generations.

Architecture

lake-meta owns an opaque generation plus a monotonic authority marker. Conditional registry create/delete and generation replacement are one atomic backend operation; version-only writes remain independent. Before explicit rollout finalization, catalogs always scan. After finalization they compare the point-read generation, scan only on change, and validate the generation again before publishing a candidate Arc<CatalogGeneration>.

Tasks

1. Add RED contract tests for atomic signaled mutations in RocksDB and Dynamo, leader fencing, authority finalization, steady point-read refresh, append churn, legacy compatibility, and scan races.
2. Add a typed signaled conditional mutation to MetaStore; implement it as a RocksDB write batch and DynamoDB transaction, and compose it with the existing lease guard in FencedMetaStore.
3. Route registry create/delete through the atomic generation signal while keeping version and incarnation updates generation-neutral.
4. Add explicit monotonic finalization with both rollout acknowledgements.
5. Teach LakeCatalog to cache authority/generation, skip unchanged scans, and reject a scan candidate when its generation moves before publication.
6. Document rollout, rollback boundary, cost model, metrics interpretation, and Kubernetes procedure.
7. Run lane-1 lifecycle, LocalStack integration, strict Clippy, full gate, rustdoc, and independent correctness/performance/release review.