Route bounded semantic actions from native accessibility peers #545

Parent: #262. Provider prerequisite: #541/#542. Platform consumer: SceneTech/AppScene#30.

Add a bounded generation-stamped native request ABI for semantic actions used by accessibility peers: focus, press, toggle, increment/decrement, set value, and set text selection where the target's DOM semantics permit them. Route work on the document worker, reject stale document/node generations, bound queues and payloads, preserve event/default-action semantics, and retire pending work on navigation or teardown. No platform objects or assistive-technology APIs belong in WebScene.

Acceptance includes authored C ABI/native/browser contracts for validity, cancellation/order, stale identity, caps, 10,000 requests, and repeated navigation/teardown. Execution and packaged assistive-technology acceptance remain in the deferred validation lane.