perf(tui): instrument scroll fast-path decline reasons

Adds scrollFastPathStats counters to render-node-to-output.ts: captures
every time a ScrollBox's DECSTBM scroll hint is generated, records
whether the fast path took it (blit+shift from prevScreen) or declined,
and why. Exposed through hermes-ink's public exports and snapshotted on
every FrameEvent so the profiler harness can correlate decline reasons
with the actual patch/renderer cost per frame.

This is pure observation — no behaviour change. Preparing for the
virtual-history rewrite: the hypothesis was that our topSpacer/
bottomSpacer scheme disqualifies every scroll via heightDelta
mismatch, but the data shows the fast path is actually taken on most
scrolls (19/23 over a 6s PageUp hold through 1100 messages) — the
remaining steady-state renderer cost is Yoga tree traversal, not
the per-frame full redraw I initially suspected.

Declines that do happen correlate with React commits that changed the
mounted range mid-scroll (heightDelta=±3 to ±35). Those are the rarer
cases the virtualization rewrite still needs to address.

No test diffs — instrumentation-only.  Build verified: `tsc --noEmit`
plus the full `npm run build` compiler post-pass pass cleanly.

ui-tui/src/lib/perfPane.tsx

@@ -44,6 +44,7 @@ import { homedir } from 'node:os'
 import { dirname, join } from 'node:path'
 
 import type { FrameEvent } from '@hermes/ink'
+import { scrollFastPathStats } from '@hermes/ink'
 import { Profiler, type ProfilerOnRenderCallback, type ReactNode } from 'react'
 
 const ENABLED = /^(?:1|true|yes|on)$/i.test((process.env.HERMES_DEV_PERF ?? '').trim())
@@ -122,8 +123,29 @@ export const logFrameEvent = ENABLED
         return
       }
 
+      // Snapshot the fast-path counters each frame. Cumulative values —
+      // consumers diff pairs to get per-frame deltas. Written verbatim
+      // so we can also see "last*" fields (which decline reason fired,
+      // and what the height math looked like).
+      const fastPath = {
+        captured: scrollFastPathStats.captured,
+        taken: scrollFastPathStats.taken,
+        declined: {
+          heightDeltaMismatch: scrollFastPathStats.declined.heightDeltaMismatch,
+          noHint: scrollFastPathStats.declined.noHint,
+          noPrevScreen: scrollFastPathStats.declined.noPrevScreen,
+          other: scrollFastPathStats.declined.other
+        },
+        lastDeclineReason: scrollFastPathStats.lastDeclineReason,
+        lastHeightDelta: scrollFastPathStats.lastHeightDelta,
+        lastHintDelta: scrollFastPathStats.lastHintDelta,
+        lastPrevHeight: scrollFastPathStats.lastPrevHeight,
+        lastScrollHeight: scrollFastPathStats.lastScrollHeight
+      }
+
       writeRow({
         durationMs: round2(event.durationMs),
+        fastPath,
         flickers: event.flickers.length ? event.flickers : undefined,
         phases: event.phases
           ? {

ui-tui/src/types/hermes-ink.d.ts

@@ -101,6 +101,24 @@ declare module '@hermes/ink' {
   export const TextInput: React.ComponentType<any>
   export const stringWidth: (s: string) => number
 
+  export type ScrollFastPathStats = {
+    captured: number
+    taken: number
+    declined: {
+      noPrevScreen: number
+      heightDeltaMismatch: number
+      noHint: number
+      other: number
+    }
+    lastDeclineReason?: string
+    lastHeightDelta?: number
+    lastHintDelta?: number
+    lastScrollHeight?: number
+    lastPrevHeight?: number
+  }
+  export const scrollFastPathStats: ScrollFastPathStats
+  export function resetScrollFastPathStats(): void
+
   export function render(node: React.ReactNode, options?: NodeJS.WriteStream | RenderOptions): Instance
 
   export function useApp(): { readonly exit: (error?: Error) => void }