add generation garbage collection

[SSmallMonster]

Summary

• add a Redis-backed durable retirement queue with leased GC work items
• retire metadata before delayed physical reclamation for overwrite, delete, and access-triggered TTL expiry
• keep generation identity checks before deleting retired layouts and retry transient cleanup failures
• cover queue behavior, local reclamation, and two-node distributed reclamation

Validation

• cargo test --features metrics --lib: 69 passed
• cargo test --test local_cluster_e2e --no-run: passed
• make e2e could not run locally because redis-server is not installed; GitHub Actions installs it for the existing E2E job