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, 10 insertions, 0 deletions

diff --git a/src/app.css b/src/app.css
index f5b269c..e5e2a99 100644
--- a/src/app.css
+++ b/src/app.css
@@ -127,3 +127,13 @@ body,
 body {
   overflow: hidden;
 }
+
+/* Transcript rows: skip layout + paint for rows scrolled out of view. A long
+   turn has hundreds of rows; without this every one is laid out on first
+   paint (and on every resize). `contain-intrinsic-size: auto <h>` makes the
+   browser remember each row's last rendered height, so scrolling back up
+   doesn't jump; the 64px is only the estimate for never-rendered rows. */
+.chat-row {
+  content-visibility: auto;
+  contain-intrinsic-size: auto 64px;
+}