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.

tests/render-composer.test.tsx

@@ -57,13 +57,19 @@ describe("Composer", () => {
 	});
 });
 
-// The Enter decision: stop a busy stream, otherwise send. (The plan-verdict
-// accept branch died 2026-09-26 with the verdict itself — the post-plan
-// reply is a natural text message.) Pure function so the static-render
-// harness can pin it (key events can't be simulated in renderToStaticMarkup).
+// The Enter decision (composer steering, 2026-10-03): while a turn runs,
+// typed text makes Enter a STEER; Enter with an empty composer stays the
+// stop shortcut. (The plan-verdict accept branch died 2026-09-26 with the
+// verdict itself — the post-plan reply is a natural text message.) Pure
+// function so the static-render harness can pin it (key events can't be
+// simulated in renderToStaticMarkup).
 describe("enterAction", () => {
-	it("busy stream: Enter stops", () => {
-		assert.equal(enterAction({ busy: true, canSend: true }), "stop");
+	it("busy stream with text: Enter steers (the queued send)", () => {
+		assert.equal(enterAction({ busy: true, canSend: true }), "send");
+	});
+
+	it("busy stream, empty composer: Enter stops", () => {
+		assert.equal(enterAction({ busy: true, canSend: false }), "stop");
 	});
 
 	it("idle: send when there is text, nothing when there is not", () => {