Part of #169. Follows #193.

• Add a package-private synchronous epoch-transition hook so shared catalog work can retire before session revision listeners observe an accepted epoch.
• Preserve deterministic prepare/commit/dispatch ordering for provider invalidations and higher response epochs.
• Tighten provider cleanup and transition dispatch to an exact undefined contract, with fail-closed quarantine for invalid returns.
• Dispose before inspecting invalid thenables, closing the hostile getter reentrancy window identified after feat(vnext): coordinate catalog epochs #193 merged.
• Keep the no-hook and null-transition paths allocation-free and add a configured-hook storm benchmark.
• Extend ADR 0005, type fixtures, lifecycle tests, and adversarial Promise/thenable coverage.

Summary by cubic

Adds a package-private, synchronous epoch-transition hook that runs after epoch install to retire catalog work before session listeners. Enforces exact-return contracts and fail-closed handling to harden reentrancy, cleanup, and poisoned-thenable edge cases; supports #169.

• New Features

  • Added SqlCatalogEpochTransitionTarget that may return a dispatch closure or null; dispatch must return exactly undefined (sync only).
  • Preserved strict ordering: transition-prepare → revision-prepare → producer settle → transition-dispatch → revision-dispatch; reentrant dispatch queues behind current dispatch.
  • Snapshots the revision audience before transition; producers/aborts see prepared revisions; listeners see already-retired work.
  • Rejects non-function transition targets during coordinator creation (reason: "invalid-transition-target"); any thrown preparation, thrown dispatch, or non-undefined return disposes fail-closed and discards response decisions.
  • Drains detached Promise/thenable results for cleanup and transition returns after disposal, avoiding hostile getter reentrancy; no-hook and null-dispatch paths allocate nothing. Updated ADR 0005, added a configured-hook storm benchmark, and expanded adversarial tests.

• Migration

  • Update SqlCatalogSubscriptionCleanup to be synchronous and return undefined only (no Promises or other return values).

^{Written for commit 9d7c9c3. Summary will update on new commits.}