perf(chat): fast open of long in-flight turns

- Batch live chat events: fold a burst in one pass per animation frame
  (rAF + timeout fallback for hidden tabs) instead of one reactive cycle per
  WS frame. Flushed before send/resync to keep ordering.
- De-dup the late-join replay against committed history: provisional chunks
  of the in-flight turn already persisted (CR-6 per-step persistence) are
  hidden by aligned-prefix matching (fail-safe: mismatch shows, never hides).
- Memoize chunks/turnMetrics getters by state identity.
- LazyCollapse: tool/thinking card bodies mount only on first open.
- content-visibility:auto on transcript rows.
- scripts/probe-replay.ts + bench-pure.ts for measuring the replay path.

1 files changed, 36 insertions, 0 deletions

diff --git a/scripts/probe-replay.ts b/scripts/probe-replay.ts
new file mode 100644
index 0000000..8a7faf9
--- /dev/null
+++ b/scripts/probe-replay.ts
@@ -0,0 +1,36 @@
+/**
+ * Probe: subscribe to a conversation over WS, capture the late-join replay,
+ * and report event counts / bytes / time-to-receive. Saves to /tmp/replay-<id>.json.
+ *   bun scripts/probe-replay.ts <conversationId> [wsUrl]
+ */
+const conv = process.argv[2];
+const url = process.argv[3] ?? "ws://localhost:24990";
+if (!conv) throw new Error("conversation id required");
+
+const ws = new WebSocket(url);
+const events: unknown[] = [];
+let bytes = 0;
+let t0 = 0;
+let lastAt = 0;
+ws.onopen = () => {
+  t0 = performance.now();
+  ws.send(JSON.stringify({ type: "chat.subscribe", conversationId: conv }));
+};
+ws.onmessage = (ev) => {
+  const s = String(ev.data);
+  const m = JSON.parse(s);
+  if (m.type !== "chat.delta") return;
+  bytes += s.length;
+  lastAt = performance.now();
+  events.push(m.event);
+};
+setTimeout(
+  async () => {
+    ws.close();
+    const counts: Record<string, number> = {};
+    for (const e of events as { type: string }[]) counts[e.type] = (counts[e.type] ?? 0) + 1;
+    console.log({ events: events.length, bytes, recvMs: Math.round(lastAt - t0), counts });
+    await Bun.write(`/tmp/replay-${conv}.json`, JSON.stringify(events));
+  },
+  Number(process.env.WAIT_MS ?? 3000),
+);