feat(tui): port claude-code's wheel accel state machine

Replaces the static WHEEL_SCROLL_STEP=1 multiplier on wheel events
with an adaptive accel state machine that infers user intent from
inter-event timing.

Algorithm ported straight from claude-code's
src/components/ScrollKeybindingHandler.tsx.  All tuning constants,
the native/xterm.js path split, the encoder-bounce detection, the
trackpad-burst signature → all theirs.  This file is a mechanical
port into our module structure.

What it does:

  precision click (>500ms gap)   1 row/event   (deliberate scan)
  sustained mouse (40-200ms)     2-6 rows      (decay curve)
  detected wheel bounce          ramps to 15   (sticky wheel-mode)
  trackpad flick (5+ <5ms)       1 row/event   (burst detect)
  direction reversal             reset to base

Two implementation paths:

  * native terminals (ghostty, iTerm2, Kitty, WezTerm) — linear
    window-ramp + optional wheel-mode curve triggered by detected
    encoder bounce.  SGR proportional reporting handled via the
    burst-count guard.

  * xterm.js (VS Code / Cursor / browser terminals) — pure
    exponential-decay curve with fractional carry.  Events arrive
    1-per-notch with no pre-amplification, so the curve is more
    aggressive.

Selected at construction via isXtermJs() from @hermes/ink (now
exported).  Per-user tune via HERMES_TUI_SCROLL_SPEED (alias
CLAUDE_CODE_SCROLL_SPEED for portability).

13 unit tests covering direction flip/bounce/reversal, idle
disengage, trackpad-burst disengage, frac invariants, and the
native vs xterm.js branches.

Profiled under --rate 30 (stress test) and --rate 10 (realistic
sustained scroll): accel ramps to cap=6 at 30Hz burst, decays to
1-3 rows at sparse 10Hz clicks.  Perf is comparable to baseline
because accel IS multiplying step — the win is perceptual (fast
flicks cover distance, slow clicks keep precision), not raw fps.

Companion to the earlier WHEEL_SCROLL_STEP=1 change: that set the
base; this modulates around it.

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

@@ -20,6 +20,7 @@ export { useTabStatus } from './ink/hooks/use-tab-status.js'
 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 { isXtermJs } from './ink/terminal.js'
 export { default as measureElement } from './ink/measure-element.js'
 export {
   resetScrollFastPathStats,