fix(desktop): stop chat scroll jumping by disabling native scroll anchoring

The thread renders virtualized turns in natural document flow with padding
spacers, and @tanstack/react-virtual already adjusts scrollTop itself when an
off-screen turn is measured and its real height differs from the 220px
estimate. With the browser default `overflow-anchor: auto`, native scroll
anchoring corrects that SAME size delta too, so the two double-correct and the
view lurches — most visibly with Windows mouse wheels, whose coarse notches
mount/measure several under-estimated turns per tick (Mac trackpads scroll
~1-3px/frame, keeping it sub-perceptual).

Set `overflow-anchor: none` on the thread viewport so only the virtualizer
compensates. Also adds `diag-scroll-reset.mjs`, a CDP wheel-up repro that A/B
tests the anchor behavior at runtime to confirm the fix.

apps/desktop/src/styles.css

@@ -709,6 +709,18 @@ canvas {
   font-size: var(--conversation-text-font-size);
 }
 
+/* The thread renders items in natural document flow (padding spacers, not
+   transforms) and @tanstack/react-virtual already adjusts scrollTop itself
+   when an off-screen turn is measured and its real height differs from the
+   220px estimate. The browser's native scroll anchoring (overflow-anchor:
+   auto) would adjust scrollTop for that SAME size delta, so the two
+   double-correct and the view lurches — most visibly on Windows mouse wheels,
+   whose coarse notches mount/measure several under-estimated turns per tick.
+   Opt out of native anchoring so only the virtualizer compensates. */
+[data-slot='aui_thread-viewport'] {
+  overflow-anchor: none;
+}
+
 [data-slot='aui_thread-content'] {
   max-width: var(--composer-width);
   padding-inline: 1.5rem;