Multi-Node-Spec test harness (worker-thread-based, with barrier sync)

[pathosDev]

Today cluster tests in tests/unit/cluster/* run as single-process with InMemoryTransport. Real multi-node tests are missing — multiple ActorSystem instances with real transport, in one process or worker-threads, deterministically coordinated. Akka's MultiNodeSpec is the prior art.

Strategy: worker-thread-based. We already have MessageChannelTransport (in src/cluster/transports/) for worker-thread mesh. The harness builds on that:

• Spawn N worker threads, each with its own ActorSystem.
• Wire them up via MessageChannel for cluster transport.
• Provide barrier synchronisation (enterBarrier(name) Akka-style) for deterministic 'all N nodes reached step X' coordination.
• Test-side API: runMultiNode(['node-a', 'node-b', 'node-c'], async (role) => { ... }) — body runs once per role, in its own worker.

Components:

File	Task
src/testkit/MultiNodeSpec.ts (new)	harness API: runMultiNode, MultiNodeProbe, barriers
src/testkit/internal/MultiNodeRunner.ts (new)	spawn workers, route messages, collect results
tests/unit/testkit/MultiNodeSpec.test.ts (new)	self-tests of the harness
tests/multi-node/*.test.ts (new)	three real multi-node tests (sharding-rebalance, pubsub-cross-node, singleton-failover)

Estimate: 4-6 days. No prerequisites. Foundation for #3 (sharding hardening) and #5 (CRDTs / replicated ES) — those depend on this landing first.

Verification — three concrete multi-node tests:

1. Sharding rebalance: 3 nodes, one fails, shards rebalance, all entities reachable.
2. PubSub cross-node: 3 nodes; subscribe on A, publish on B, receipt on C.
3. Singleton failover: 3 nodes; singleton runs on oldest, oldest dies, failover to next-oldest.

See the roadmap plan for full context (item 2 of 5).

[pathosDev]

Closed by 317ca08. Note: the title says "worker-thread-based" but we shipped the simpler in-process variant — every role runs in the same event loop, routed through a process-global MultiNodeTransport. Trade-off: no true parallelism, but determinism, fast test starts, and clean stack traces. A worker-thread follow-up can re-use the same public API.

Delivered:

• src/testkit/MultiNodeSpec.ts — public harness
• src/testkit/internal/MultiNodeTransport.ts — in-process transport with bidirectional partition controls
• tests/multi-node/pubsub-cross-node.test.ts — 2 tests
• tests/multi-node/singleton-failover.test.ts — 1 test
• tests/multi-node/sharding-rebalance.test.ts — 1 test
• tests/unit/testkit/MultiNodeSpec.test.ts — 10 self-tests

All 14 tests green; full suite still green at 1105 pass / 10 skip / 0 fail.