Add native keyboard scrolling to the chat history

[neo773]

What

Keyboard control for the transcript, tuned to feel like a native macOS scroll view — no one should be able to tell this is a custom GPUI renderer.

• ↑/↓ — tap for one crisp line step (~50ms, Chrome-parity); hold for continuous scrolling that ramps to cruise speed and stops on key-up. OS key-repeats are treated as hold-liveness signals, not scroll quanta, so there's no line…pause…line-line stutter.
• ⌥↑/⌥↓ — glide between segments (the rail's user-prompt ticks), riding the same animation as a rail-tick click. Past the last segment it hands off to the scroll-to-bottom spring and re-engages autoscroll.
• PageUp/PageDown — page scrolling everywhere, even while typing (Messages-style). Home/End, ⌘↑/⌘↓ — top/bottom when the chat holds focus.
• Focus handles itself — click into the chat to scroll with bare arrows; arrows fall through from an empty composer; typing while the chat is focused hops back to the composer with the keystroke; Escape returns to the composer. ⌘C still copies the markdown selection.

How

• Every keyboard animation is frame-driven from render via window.on_next_frame — vsync-locked, 120Hz on ProMotion — never wall-clock timer loops. The rail-click glide moved onto the same driver.
• The row glide gets an overtime landing: when its fixed timeline expires short (tall unmeasured rows, e.g. 20–40 line prompts), it closes the remainder with a quick exponential tail instead of hard-snapping.
• The stick-to-bottom spring gets a terminal phase (minimum closing velocity) so its asymptotic tail can't leave "at bottom" consumers (rail active tick) lagging a visually finished scroll; it lands in the glued representation the app already keys off.
• Bottom re-pinning is intent-based: only deliberate downward keyboard motion re-engages the pin — upward motion never can. Wheel input always cancels any keyboard/rail glide.
• Reduced-motion snaps instantly throughout.

Testing

• cargo test -p comet-ui: 313 passed.
• Hand-tested feel: tap/hold arrows from every state (pinned bottom, mid-history, top edge), segment jumps across tall prompts, wheel interrupts, streaming autoscroll handoff, empty vs non-empty composer, focus round-trips.

🤖 Generated with Claude Code

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