streaming: connection diet — the parallel-session freeze, fixed

Root cause (postmortem, verified live): Chromium caps 6 concurrent
HTTP/1.1 connections per origin. Every running turn pinned slots for
its whole life — the POST /api/chat response stream plus a reattach
GET stream per visit that never closed on navigate-away — so two
concurrent thinking sessions starved navigation RSC fetches in the
browser queue (147s measured) until the turns settled. Server was
exonerated (3ms curls during the freeze); main thread idle throughout.

- COMPACTION LAW + WIRE DIET (ui-stream): store the states, not the
  chunks — replace-in-place snapshot retention keyed by updateSlots,
  ~100ms coalesced latest-wins fan-out, slim wire drops pi's duplicate
  assistantMessageEvent payload.
- DIET B: POST /api/chat acks and closes ({ok:true}) — the turn wire
  is GET /stream for everyone, submitter included; send() reattaches
  via refetch-first resumeStream(true).
- DIET A: unmount cleanup calls model.stop() — navigating away aborts
  the view connection (turns are server-side background objects).
- Answer-flicker fix: resumeStream's eager tail clear moved off the
  refetch path (the refetch swaps base+tail atomically; clean-slate
  guard restored on refetch failure) — the optimistic answer card
  survives refetch latency (1.7s stalls measured on dev).
- Tests converted to the ack contract (drainTurn/drainRunning helpers,
  dietFetch mock) + a pin that the POST body reads while the turn runs.

Gates: 1489/1489, typecheck, lint. Live: dual streaming sessions,
navigations 335-648ms, in-page settings 19ms (was 147s).

tests/demos.test.tsx

@@ -59,6 +59,7 @@ import { createTeacherTools } from "../src/agent/tools.ts";
 import { chatStore } from "../src/store/chats.ts";
 import { jobStore } from "../src/store/jobs.ts";
 import { handleChatRequest } from "../src/server/chat-handler.ts";
+import { runningTurn } from "../src/server/turn-runs.ts";
 import { createTeacherAgent } from "../src/agent/teacher-agent.ts";
 import { projectView as projectNative } from "../src/core/view.ts";
 import type { PiEngine } from "../src/agent/models.ts";
@@ -786,6 +787,15 @@ async function drain(res: Response): Promise<string> {
 	return res.text();
 }
 
+/** CONNECTION DIET B (owner, 2026-10-10): the POST acks and closes; drain
+ *  the turn's wire through the reattach stream instead (settle-tolerant:
+ *  a short turn may finish inside the handler). */
+async function drainRunning(chatId: string): Promise<string> {
+	const handle = runningTurn(chatId);
+	if (!handle) return ""; // the turn already settled — completion happened
+	return new Response(handle.subscribe()).text();
+}
+
 
 describe("demo/visual job runners", () => {
 	it("runJob completes a demo: result on the job, steer via deliver, ack stays on the part", async () => {
@@ -931,34 +941,34 @@ describe("demo/visual job runners", () => {
 		const { steers, runner } = testRunner();
 
 		try {
-			await drain(
-				await handleChatRequest(
-					new Request("http://test/api/chat", {
-						method: "POST",
-						headers: { "Content-Type": "application/json" },
-						body: JSON.stringify({
+			const ack = await handleChatRequest(
+				new Request("http://test/api/chat", {
+					method: "POST",
+					headers: { "Content-Type": "application/json" },
+					body: JSON.stringify({
+						chatId: chat.id,
+						// Wave B: the input-events contract — one event,
+						// never a client transcript.
+						input: { type: "text", text: "teach me compounding" },
+					}),
+				}),
+				// The handler hands the factory the native build options
+				// (messages are the merged record; the record IS the agent
+				// state — no adapter conversion).
+				(opts) =>
+					createTeacherAgent(undefined, {
+						engine: teacher,
+						messages: opts.messages,
+						tools: createTeacherTools({
 							chatId: chat.id,
-							// Wave B: the input-events contract — one event,
-							// never a client transcript.
-							input: { type: "text", text: "teach me compounding" },
+							chatStore,
+							jobStore,
+							spawn: (job, input) => void runner.runJob(job.id, input),
 						}),
 					}),
-					// The handler hands the factory the native build options
-					// (messages are the merged record; the record IS the agent
-					// state — no adapter conversion).
-					(opts) =>
-						createTeacherAgent(undefined, {
-							engine: teacher,
-							messages: opts.messages,
-							tools: createTeacherTools({
-								chatId: chat.id,
-								chatStore,
-								jobStore,
-								spawn: (job, input) => void runner.runJob(job.id, input),
-							}),
-						}),
-				),
 			);
+			if (ack.status !== 200) assert.fail(`expected 200 ack, got ${ack.status}: ${await ack.text()}`);
+			await drainRunning(chat.id);
 
 			const job = jobStore.findJobByToolCallId(chat.id, "call-d2");
 			assert.ok(job, "dispatch created a job at tool-execute time");