Scale the agentic runtime to distributed workers and messaging

## Origin

Deferred from the MVP proposed as a fresh realization of #485.

## Goal

Scale the single-machine durable runtime into a multi-process and eventually distributed worker system without changing its event-sourced execution semantics.

## Scope

- Replace local worker leases/notifications with database- and queue-backed primitives.
- Define worker identity, heartbeats, lease expiry, orphan recovery, backpressure, and fair scheduling.
- Support direct typed addressing in addition to parent/child routing, with authorization and hop/fan-out limits.
- Add cancellation and interruption at deterministic checkpoints.
- Detect deadlock/livelock and expose operator diagnostics.
- Load-test hundreds of concurrent logical nodes and multiple simultaneous runs.

## Prerequisite

The MVP must first establish replay, idempotency, message-boundary, and state-transition semantics on one machine. Distribution must preserve those contracts rather than define new ones accidentally.

## Acceptance criteria

- [ ] Multi-worker execution produces the same projections as the single-process reference runner.
- [ ] Worker death, duplicate delivery, delayed messages, and lease theft are covered by fault-injection tests.
- [ ] Backpressure and budget limits remain enforceable under concurrency.
- [ ] A run can be cancelled and resumed without duplicate completed effects.
- [ ] Throughput/latency curves and operational limits are documented.