composer steering: the general steering machinery (owner arc 2026-10-03)

STEERING, NOT FOLLOW-UPS — pi-agent-core's two rails, used deliberately:
agent.steer() injects at the next response boundary INSIDE the live run
(one logical turn, the model changes course); followUp() would only extend
an ended run — not built. The same rail job markers already ride
(steeringMode 'all', the 2026-09-26 doctrine).

- POST /api/chats/[id]/steer: 404/400 honest guards; no live turn → 409
  and the client silently falls back to a plain text send (the settle
  race); live → steerLiveChat hands the message to the live agent's
  steering queue AND parks it in the userSteers registry — the stranding
  safety net. A steer stranded behind a question termination or abort
  stays parked and rides the chat's NEXT run (handOffEligible), the same
  deliver-or-requeue lifecycle as job markers. Reconciliation at settle:
  delivered ids drop (reconcileUserSteers).
- Client: steer(text) queues + POSTs without flipping any turn state;
  the drain's verbatim message events upsert the real user card by the
  client-minted id (steer_<localid>) and the indicator fold drops the
  line. canSend no longer gates on turnInFlight — Enter mid-turn steers;
  Enter with an empty composer still stops; the square button stays Stop.
- The queued-steer indicator (owner spec): quiet one-liners above the
  composer shell — right-aligned to where its corner rounding starts,
  truncated with an ellipsis, fixed max width, ~half opacity, no
  background, no container. Tunable later.
- Verified from pi-agent-core source: steering drains per completed turn
  inside the loop; injected messages emit message_start/end verbatim;
  finishTurn {action:'end'} (our question stopper) exits BEFORE the
  steering poll — hence the registry.

components/composer.tsx

@@ -18,7 +18,11 @@ import { Hint } from "@/components/ui/hint";
  *  can pin the decision (key events can't be simulated in
  *  renderToStaticMarkup). */
 export function enterAction(state: { busy: boolean; canSend: boolean }): "stop" | "send" | "none" {
-	if (state.busy) return "stop";
+	// Composer steering (owner arc 2026-10-03): while a turn runs, typed
+	// text makes Enter a STEER (the queue-and-redirect send); Enter with
+	// an empty composer stays the stop shortcut. The button itself keeps
+	// the square — Stop stays one click away at all times.
+	if (state.busy) return state.canSend ? "send" : "stop";
 	if (state.canSend) return "send";
 	return "none";
 }