[lifecycle-RSK] RSK-13: Wrap run state in a task actor with a serialized mailbox #1806

Tracking

• Epic: [lifecycle-RSK] Run-state kernel #1790
• Size: 1 SP
• Blocked by: [lifecycle-RSK] RSK-07: Flip abort and abortReason reads #1797, [lifecycle-RSK] RSK-08: Flip abandoned reads #1798, [lifecycle-RSK] RSK-11: Flip isInitialized reads #1799, [lifecycle-RSK] RSK-16: Flip stream and turn-field reads #1805, [lifecycle-RSK] RSK-19: Keep the first abort reason #1802, [lifecycle-RSK] RSK-20: Check abort before an ask posts #1803
• Design doc: docs/architecture/task-lifecycle-target-architecture.md (not yet on main)
• Ticket plan: docs/architecture/task-lifecycle-run-state-kernel-tickets.md (not yet on main)

Context

The real hazard is interleaving between await points, not concurrent mutation. The extension host is single-threaded. The actor is a serialized mailbox that processes one event at a time.

Developer Notes

• Precondition: [lifecycle-RSK] RSK-07: Flip abort and abortReason reads #1797, [lifecycle-RSK] RSK-08: Flip abandoned reads #1798, [lifecycle-RSK] RSK-11: Flip isInitialized reads #1799, and [lifecycle-RSK] RSK-16: Flip stream and turn-field reads #1805 merged, so the kernel is the only run-state source.
• Replace the write helpers with a mailbox on Task: accept a RunEvent, apply nextRunState, keep ok, report rejected.
• State the rule: an await inside a handler does not hold the mailbox. Re-read state after each await.
• Reuse the runDelegationTransition queue (ClineProvider.ts:148-172) and the abortPromise ??= guard (Task.ts:2687). Do not add a new lock.
• A rejected event must not mutate state.
• Log each accepted and rejected transition with taskId and generation.

Exit criteria: internal callers drive run state through the mailbox. Tests pass.

Reversibility: internal to Task.