B2 — Machine definition + local ActorHost kernel

Implement B2 of plans/cleanup-state-machines.md ("Phase B — B2"). The plan wins; the detailed design source is plans/distrbuted-machines.md ("Machine definition API", "Actor host", "Dispatch tickets", "Local actor references", "Lifecycle policy") — read both before coding. Prereq: B0 ADR (lifecycle matrix, intent classes).

Scope (synthetic machines only — no production domain migrates here):

1. DistributedMachineDefinition: pure transition + runtime metadata + host placement + scheduler/command-runner factories + lifecycle policy; optional persistence/remote contracts are TYPES only in this PR (implementations come with B3/C waves). Avoid elaborate type-level inference — clear generic annotations over a clever DSL (the distributed plan says this explicitly; honor it).
2. Actor identity: actor instance ID (new per lifetime), snapshot revision (increments only on snapshot reference change), transaction sequence (every processed event including ignored). These compose the existing InvocationRef; they do not replace it.
3. ActorHost: keyed ensure/peek/disposeKey/disposeMachine/dispose over the existing TransactionalDispatcher, TaskScope, TimerLeaseScope, and trace infrastructure — the distributed layer COMPOSES src/state_machines/ primitives, never duplicates them (boundary test asserting this).
4. Dispatch tickets: dispatcher.enqueue(event) → ticket settling applied/ignored/failed/disposed for its exact FIFO entry, per the distributed plan's requirements list (re-entrant enqueue settles on its own turn; disposal settles all unprocessed; command failure after commit never rewrites an applied ticket; send() remains the ticket-discarding wrapper). This modifies the shared dispatcher — isolate it in its own commit with the dispatcher's conformance suite extended first.
5. Local actor refs (getSnapshot/subscribe/send, synchronous enqueue preserved) + the selector-aware hooks from A1 working against them.
6. Actor lifecycle policies from the matrix (subscription-creates, dispatch-creates, idle eviction, terminal retention, entity-deletion, shutdown flush) + the ordered disposal sequence (stop admission → settle waiters → terminal projection → cancel tasks/timers → flush → unsubscribe → dispose dispatcher).
7. Host conformance suite covering the distributed plan's "Local host conformance" list, runnable against any hosted actor.

Verify: typecheck, full unit tests (dispatcher conformance + new host conformance + negative tests), lint, golden suite untouched-and-green. /deep-review. Branch cleanup-b2-actor-kernel; /pr-push; update plan status. Separate revert point: nothing outside src/state_machines/ and the new module depends on this PR yet.