perf(desktop): fix "Enter jumps up" on long threads

User reported: after pressing Enter on a long thread, the view jumps up
— the just-submitted message disappears below the fold. Confirmed via
apps/desktop/scripts/measure-jump.mjs:

  before:  distFromBottom 0 → 49.5px, sticks there permanently
  after:   distFromBottom 0 → ~0 (worst case 4px for one frame)

Root cause in useThreadScrollAnchor (thread-virtualizer.tsx):

1. The sticky-bottom logic disarmed on any scroll event where
   `scrollTop < lastTopRef.current`. That check can't distinguish a
   user scrolling up from a programmatic `pinToBottom` write that
   the browser clamped short of bottom (because content also grew in
   the same frame, so `scrollTop = scrollHeight` lands at
   `scrollHeight - clientHeight` for the OLD scrollHeight, which is
   now below the NEW scrollHeight). Result: sticky-bottom disarmed
   permanently on the user's first submit.

2. There was no synchronous pin tied to React's commit phase. By the
   time the ResizeObserver fired and re-pinned, the user had already
   seen ~50ms of "message below the fold" — visually that reads as the
   view jumping up.

Fix:

- `programmaticScrollPendingRef` counter tracks scroll events we
  expect to be ours (one per `pinToBottom` write). The scroll handler
  skips the disarm check when consuming a pending tick, keeps the
  arm bit true, and re-pins synchronously if the browser clamped us
  short of bottom. A depth cap (8) breaks runaway loops in
  pathological streaming-burst layouts.

- `useLayoutEffect` on `groupCount` increase pins BEFORE the browser
  paints, eliminating the visible ~50ms window between optimistic
  user-message insert and the RO/scroll-event chain firing.

Verified on the long Cloud Shadows thread (7-8 turns, ~11k px tall):
all three repro runs now hold within 0–4 px of bottom across the
post-Enter transition. Submit latency unchanged (paint 77–107 ms),
streaming-typing latency unchanged.

Also adds three debug harnesses:
  - measure-jump.mjs   — sample thread scroll across Enter
  - probe-thread.mjs   — dump current thread / scroll state
  - diag-jump.mjs      — intercept scrollTop + RO + mutations across Enter

apps/desktop/scripts/probe-thread.mjs

@@ -0,0 +1,40 @@
+// Probe the cloud shadows thread state — count messages, turn pairs,
+// thread height, composer state
+const list = await (await fetch('http://127.0.0.1:9222/json/list')).json()
+const tgt = list.find(t => t.type === 'page' && t.url.startsWith('http'))
+const ws = new WebSocket(tgt.webSocketDebuggerUrl)
+let id = 0
+const pending = new Map()
+ws.addEventListener('message', ev => {
+  const m = JSON.parse(ev.data)
+  if (m.id != null && pending.has(m.id)) {
+    pending.get(m.id)(m)
+    pending.delete(m.id)
+  }
+})
+await new Promise(r => ws.addEventListener('open', r))
+const send = (m, p = {}) =>
+  new Promise(r => {
+    const i = ++id
+    pending.set(i, r)
+    ws.send(JSON.stringify({ id: i, method: m, params: p }))
+  })
+
+const r = await send('Runtime.evaluate', {
+  expression: `JSON.stringify({
+    url: location.href,
+    title: document.title,
+    turnPairs: document.querySelectorAll('[data-slot="aui_turn-pair"]').length,
+    assistantMsgs: document.querySelectorAll('[data-slot="aui_assistant-message-root"]').length,
+    userMsgs: document.querySelectorAll('[data-message-role="user"], [data-slot="aui_user-message-root"]').length,
+    totalDomNodes: document.querySelectorAll('*').length,
+    threadViewportScrollHeight: document.querySelector('[data-slot="aui_thread-viewport"]')?.scrollHeight ?? null,
+    threadViewportClientHeight: document.querySelector('[data-slot="aui_thread-viewport"]')?.clientHeight ?? null,
+    threadViewportScrollTop: document.querySelector('[data-slot="aui_thread-viewport"]')?.scrollTop ?? null,
+    composer: !!document.querySelector('[data-slot="composer-rich-input"]'),
+    busy: !!document.querySelector('[aria-label*="Stop"]')
+  })`,
+  returnByValue: true
+})
+console.log(JSON.parse(r.result.result.value))
+ws.close()