[Feature] Public persistence testkit: journal/snapshot conformance suite + failure injection

[pathosDev]

Use case

The repo has a thorough internal cross-backend contract suite for journals/snapshot stores, but it is not exported — users who implement a custom backend (the plugin registry explicitly supports this) have no way to prove conformance, and no failure-injection tools to test their actors against journal errors. Pekko invests exactly here (persistence testkit read journal, JournalPersistFailed / JournalPersistRejected signals) because it is what makes third-party backends viable.

Proposed approach

Export from actor-ts/testkit:

• verifyJournalContract(() => new MyJournal(options)) — the parameterized conformance suite (ordering, sequence numbers, tags, deletes, concurrency).
• The same for snapshot stores and durable-state stores.
• A FailureInjectingJournal decorator: scripted failures (persist rejected, persist failed, read timeout, partial batch) to test actor recovery behavior.

Acceptance criteria

• Contract suites exported and documented (EN+DE) with a "write your own backend" guide
• The in-repo backends run through the exported suite (single source of truth, no drift)
• Failure-injection decorator with deterministic scripting via the testkit scheduler

Related

• [Feature] Unify persistence plugin registration (self-activating, top-level url, uniform handles, SQLite plugin, ConfigKeys) #386 (plugin registration unification) — the registration side of the same story
• [Feature] Persistence contract polish: integrity hook, InMemory snapshot keepN, prune policy #493 (persistence contract polish)