feat(client): pixel-smooth scrollback scrolling

[jesse23]

Description

Trackpad scrolling stepped a whole line at a time behind a 100 ms easing animation, and in mouse-tracking apps it ran far ahead of the fingers.

Scrollback (normal screen, no mouse tracking). ghostty-web's viewport is whole lines only. src/client/smooth-scroll.ts keeps the scroll position as fractional lines and takes over wheel events:

• The whole part goes to ghostty-web as viewportY (scrollToLine), rendered as usual.
• The fraction becomes a device-pixel-snapped CSS offset: the canvas is translated down and its bottom edge clipped, and a one-row strip above it shows the scrollback line sliding in. The strip is drawn with ghostty-web's own renderLine (renderer context swapped for the strip's), so it matches the canvas.
• At a whole-line position there is no transform and the strip is hidden, so a resting terminal renders as before.
• ghostty-web's own viewport moves (output snapping to the bottom, keyboard scrolling, scrollbar drag, resize) reset the position.

Mouse-tracking apps (herdr, Claude Code fullscreen, vim mouse=a). These scroll their own view in whole lines, so no pixel offset is possible. The SGR wheel handler sent at least one tick per wheel event, and a trackpad fires many small events plus momentum. Pixel deltas now count by distance: one tick per cell height scrolled, 1:1 with the OS like native macOS scrolling. mouseScrollSpeed no longer applies to pixel deltas (Ghostty likewise skips its multiplier for precision scrolls); it still scales line/page deltas.

Builds on #46 (uses its whole-ratio renderer DPR for the strip).

Verification

• bun run build
• bun test (441 passed, including the new smooth-scroll.test.ts)
• bun run lint
• In Chrome (DPR 2), synthetic pixel wheel events: a 6px offset renders crisply with the incoming line joined seamlessly at the top; scrolling to the bottom, new output while partly scrolled, and a viewport resize all reset the offset; less on the alternate screen is unaffected.
• Mouse-tracking app (cat -v with modes 1002/1006): 20 × 2px wheel events sent 4 SGR ticks with distance counting and mouseScrollSpeed: 1.5 (14px cells), instead of 20. With the multiplier dropped for pixel deltas, the same input gives 2 (not re-measured in the browser).
• Manually checked on a MacBook trackpad, in plain scrollback and in herdr: scrolling is smooth, and select-to-copy still works. Feedback that herdr felt slightly faster than native led to dropping mouseScrollSpeed for pixel deltas.
• Known limits while at a fractional position: the overlay scrollbar shifts by under a line, and Kitty graphics placements are not offset.

ADRs

• ADR 040 — Pixel-smooth scrollback scrolling