fix: retain stopped synthetic turns until the room resumes

[AlexanderZ-Band]

Why

A synthetic contact-hub turn can be lost when a control interrupts setup or defers admission. STOP must preserve queued work for PLAY while keeping other rooms responsive.

What

Retain synthetic events until their turn advances, and cover setup-window controls, stopped draining and settled-turn posting with runtime regressions.

Scope

Independent runtime portion of #791. Closes INT-1781; contributes to INT-1133. Does not migrate adapter backends or change bridge startup.

Backend cancellation is reviewed in #834 → #835 → #791; bridge startup is #833.

[linear-code]

INT-1781

INT-1133

[AlexanderZ-Band]

Full-PR review at 0676394d51de: no verified findings across Style & Hygiene, Design & Reuse, Runtime Safety & Observability, Test Quality, Edge-Case Flow Analysis, or Logical Bugs. Converged in cycle 1. Focused runtime regressions passed; complete unit suite is running.

[Copilot]

🟢 Approved

Static review identified no blocking issues in the retention logic, scope lifecycle, or regression coverage; tests were not executed.

No open issues 🎉

What changed in this PR

Preserves synthetic contact-hub events across STOP and deferred admission so PLAY can resume queued work without blocking other rooms. This is the runtime-only portion of the broader cancellation work.

Changes:

• Retain synthetic events until their turns advance.
• Add block-scoped turn admission and settlement.
• Cover setup-window controls, stopped draining, and stopped posting with runtime regressions.

File	Description
tests/​runtime/​test_tools.py	Use scoped turns in stopped-post checks.
tests/​runtime/​test_activity_stop.py	Update activity tests to scoped turns.
tests/​runtime/​runtimecontrol/​test_stop_notification.py	Scope stop-notification test turns.
tests/​runtime/​runtimecontrol/​test_platform_stop_hook.py	Scope platform-stop hook checks.
tests/​runtime/​resync/​test_stopped_drain.py	Verify STOP preserves queued events and other-room responsiveness.
tests/​runtime/​helpers.py	Support message metadata in test fixtures.
tests/​runtime/​executioninterrupt/​test_stopped_posts.py	Exercise stopped posting across scoped turns.
tests/​runtime/​executioninterrupt/​test_setup_window_control.py	Cover controls during setup and processed-message skips.
tests/​runtime/​executioninterrupt/​test_room_shutdown.py	Scope observer shutdown tests.
tests/​runtime/​executioninterrupt/​kit.py	Allow configurable hydration and room IDs.
src/​band/​runtime/​execution.py	Add scoped turns and retain unadvanced synthetic events.

_{Review effort: Balanced}