[lifecycle-RSK] RSK-09: Add the turn sub-machine #1800

Tracking

• Epic: [lifecycle-RSK] Run-state kernel #1790
• Size: 1 SP
• Blocked by: [lifecycle-RSK] RSK-01: Add the pure run-state kernel module #1791
• 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

Five fields are within-turn bookkeeping: isWaitingForFirstChunk, didRejectTool, didAlreadyUseTool, didToolFailInCurrentTurn, didCompleteReadingStream. A turn is one request iteration of the loop. The code writes these fields before the stream starts and after it ends (Task.ts:3228-3245, 3768, 3810, 3830, 4029). So the turn sub-machine is independent of the stream region.

Developer Notes

• Add a turn state with a turnStarted event. The event maps to the per-request reset block (Task.ts:3228-3245).
• Model the five fields as the turn state. Do not tie the turn state to stream.tag.
• Add unit tests for the turn transitions, including tool writes after the stream ends and an abort before the first chunk.
• Do not route writes or flip reads. [lifecycle-RSK] RSK-10: Route stream, ask, and turn-field writes through events #1804 and [lifecycle-RSK] RSK-16: Flip stream and turn-field reads #1805 do that.

Exit criteria: the turn sub-machine compiles and is tested. Nothing is wired.

Reversibility: additive.