test(mobile): lock the history-position hold's release path

The tail-follow positive control never drove a paging cycle, so a hold that
latched on — chat permanently unable to follow the agent, the worst failure
mode of this design — passed the whole suite. Drive the real
loadingEarlier -> prepend -> settled sequence before the append.

Also record why onContentSizeChange reads the render snapshot instead of the
ref: effects flush before native layout, so the ref is already cleared there.

mobile/src/session/MobileNativeChatView.test.ts

@@ -222,7 +222,12 @@ describe('MobileNativeChatView', () => {
   it('still follows the tail once the history page has settled', async () => {
     vi.useFakeTimers()
     try {
-      await render({ messages: [older, message], hasMore: false })
+      await render({ messages: [message], hasMore: true })
+      await act(async () => vi.runAllTimersAsync())
+      // Drive a real paging cycle first: a hold that never releases would leave
+      // the chat permanently unable to follow the agent.
+      await update({ messages: [message], hasMore: true, loadingEarlier: true })
+      await update({ messages: [older, message], hasMore: false, loadingEarlier: false })
       await act(async () => vi.runAllTimersAsync())
       scrollToEnd.mockClear()
 

mobile/src/session/MobileNativeChatView.tsx

@@ -154,6 +154,8 @@ export function MobileNativeChatView({
   // A short transcript leaves the reader both at the top and `atBottom`, so the
   // tail-follow paths would scroll away the history that just paged in.
   const keepHistoryPositionRef = useRef(Boolean(loadingEarlier))
+  // Snapshot, not a live ref read: effects flush before native layout, so by the
+  // time `onContentSizeChange` runs the ref is already cleared.
   const keepHistoryPosition = keepHistoryPositionRef.current
   const { fontScale, pinchGesture } = useMobileNativeChatPinchGesture()
   useEffect(