feat(chat): migrate history list to @tanstack/react-virtual with deterministic mobile history loading

- Replace virtua with @tanstack/react-virtual for chat history on all
  surfaces: bottom anchoring (anchorTo: end), key-stable prepend
  preservation, and native iOS touch/momentum deferral live in the core
- Patch virtual-core to clamp the render range to real scroll bounds
  during transient adjustments
- Rows render in normal flow inside a translated wrapper so sticky user
  headers keep working; measurement snapshots cached per session
- Pre-write container height in scrollToFn so the browser cannot clamp
  anchor corrections to the stale height; hold the prepend anchor for up
  to 180 frames on mobile while fresh rows settle (cancelled by user
  input; desktop relies on core anchoring alone)
- Adaptive row-size estimate from per-session measured averages; disable
  reveal fade-in for virtualized history rows
- Mobile loads older history only through an explicit localized top
  button: no scroll-position trigger and no post-mount background
  prepend, so every insert happens from a resting state; a quiet-window
  hold defers any stray prepend commit while a touch gesture is active
- Desktop/VS Code keep the seamless scroll-up trigger and progressive
  background prepend

packages/ui/src/sync/use-sync.ts

@@ -503,11 +503,15 @@ export function useSync() {
           shouldLoadMessages ? loadMessages(sessionID, { isStale }) : Promise.resolve(),
         ])
 
-        // Progressive mount on desktop: after the initial page resolves, if the
-        // session isn't stale and the server indicated more messages, dispatch a
-        // second fetch to prepend older history. Mobile avoids this background
-        // prepend because adding rows after first paint on a narrow viewport can
-        // visibly shift the timeline; user scroll still loads older history.
+        // Progressive mount (desktop/VS Code): after the initial page
+        // resolves, if the session isn't stale and the server indicated more
+        // messages, dispatch a second fetch to prepend older history — it
+        // gives the scroll container headroom so the scroll-up trigger fires
+        // seamlessly. Mobile deliberately opts out: it has no scroll-position
+        // trigger at all — ALL older history loads happen through the
+        // explicit "load older" button at the top, so every prepend lands
+        // from a resting state the user initiated. (The initial page itself,
+        // including the turn-boundary extension, is unaffected.)
         if (!isStale() && !isMobileSurfaceRuntime()) {
           const currentMeta = getMetaFor(sessionID)
           if (currentMeta.cursor && !currentMeta.complete) {