Round-4 reconciliation of #5986/#5988 with the v2 cutover files:
- PendingUserInputCard adopts main's collapsible overlay redesign with the
  v2 RuntimeRequestId/responseCapability plumbing (dead provider processes
  still read-only the card)
- ThreadFeed adopts the thread-feed-live-follow transition model, the
  user-scroll settle window, momentum handoff, and env-scoped feed keys
  while keeping the v2 nearListEnd layout gating
- ThreadDetailScreen hides (not unmounts) the composer while a user-input
  request owns the slot; multi-select answers flow as arrays end to end
  (threadActivity toggle/build helpers + tests, ThreadUserInputQuestion)
- ThreadComposer keeps the v2 canStopThread stop gate under main's
  onEditorFocusChange rename; standalone stop reuses the shared renderer
- Restored the branch's steer stop/send tests alongside main's composer
  test suite

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>