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).

app/api/chats/[id]/stream/route.ts

@@ -1,10 +1,12 @@
 /**
  * THE REATTACH STREAM (background-turn runner wave, owner ruling
- * 2026-09-25): one SSE stream per chat, same wire dialect as the POST
- * /api/chat stream. A running turn → subscribe to the live turn object:
- * FULL replay of the event log, then live chunks until settle ([DONE]).
- * No running turn → the cheap terminal ({"type":"turn-settled"} + [DONE]):
- * the record is current, the client refetches it.
+ * 2026-09-25) — and, since CONNECTION DIET B (owner, 2026-10-10), THE
+ * turn wire, period: POST /api/chat acknowledges and closes, so the
+ * submitter too attaches here. One SSE stream per chat: a running turn →
+ * subscribe to the live turn object: FULL replay of the event log, then
+ * live chunks until settle ([DONE]). No running turn → the cheap terminal
+ * ({"type":"turn-settled"} + [DONE]): the record is current, the client
+ * refetches it.
  *
  * 404 mirrors the POST handler's unknown-chat. Framework route is a thin
  * wrapper over the server barrel (the import law); tests drive the seams