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/packages/hermes-ink/src/entry-exports.ts

@@ -21,6 +21,11 @@ export { useTerminalFocus } from './ink/hooks/use-terminal-focus.js'
 export { useTerminalTitle } from './ink/hooks/use-terminal-title.js'
 export { useTerminalViewport } from './ink/hooks/use-terminal-viewport.js'
 export { default as measureElement } from './ink/measure-element.js'
+export {
+  resetScrollFastPathStats,
+  scrollFastPathStats,
+  type ScrollFastPathStats
+} from './ink/render-node-to-output.js'
 export { createRoot, default as render, renderSync } from './ink/root.js'
 export { stringWidth } from './ink/stringWidth.js'
 export { default as TextInput, UncontrolledTextInput } from 'ink-text-input'