[foxnne]

Fixes #568

What changes

On the web, an Apple Pencil is now the mouse, not a finger. iPad Safari sends a pencil as a
TouchEvent whose touch has touchType === "stylus", and web/web.js turned every touch into
a dvui touch button, so dvui.pointerKind() said .touch while drawing with a pencil: nothing
stayed hovered after a lift, and canvases panned as they do for a finger. Now a stylus touch goes
in as mouse motion to its position (in drawing-buffer pixels, as mousemove does) followed by a
left-button press or release, with the event's modifiers. It takes no touch slot. dvui then
reports .mouse, the same answer native gives once SDL's pen touches are excluded
(#563, backend/src/touch_slots.zig). Fingers, and touches with no touchType (browsers
other than Safari), are unchanged.

touchcancel now ends a touch the way touchend does. It had no handler, so a touch the system
took (a gesture, an alert) left its button held. A pencil's held mouse button would be worse,
because it keeps dragging until the next press.

The change is in fizzy's own web backend, not dvui-dev: fizzy builds dvui in custom mode and
links web/web.js + src/backend/WebBackend.zig (docs/DEPENDENCIES.md), so no dvui commit and
no repin are needed.

Considered and dropped: moving to Pointer Events and pointerType ("pen"/"touch"/
"mouse"). That would also cover Android styluses and pencil hover, but it replaces all of the
mouse and touch input paths (capture, the compatibility mouse events, the on-screen-keyboard
oskCheck that iOS only allows inside a touch handler) for a pencil fix. Worth doing as its own
change if Android stylus support is wanted.

SDK impact

• None

Verified

• A Node harness (not in the repo; fizzy has no JS test setup) loads web/web.js, runs
  Dvui.run with a stub canvas and wasm exports, and fires fake touch events at the handlers.
  A stylus start/move/end sends mouse motion at pixel coordinates, then a left press or release
  with the modifiers; a stylus takes no touch slot; a cancelled stylus releases the button.
  Finger and no-touchType touches still send touch press/motion/release at normalised
  coordinates; a cancelled finger releases its touch button. On main's web.js the stylus
  cases fail (they send touch events).
• zig build, zig build test, zig build test-integration, zig build check-web,
  zig build test-sdk-version, zig build check-tests-reached on macOS, and
  scripts/check-examples.sh with EXAMPLES_DIR (example-app zig-0.17, example-plugin main:
  example-plugin has no zig-0.17 branch any more, so the script's default clone fails on main). scripts/check-fmt.sh flags only backend/src/platform/win32_titlebar.zig, which is
  unformatted on main too (main's Format job fails since windows: a dragged view is carried outside its window #463); this PR touches no Zig.
• Web, by hand: on an iPad with an Apple Pencil in Safari, open this branch's build
  (zig build web, scripts/serve-web.sh, the Mac's address on :8765). Tap a tab or tree row
  with the pencil and lift: it stays lit (it went dark before). In a pixi canvas, a pencil stroke
  draws rather than pans. A finger tap on a tab still leaves it unlit.
• macOS / Windows / Linux: not affected (web only).

Follow-ups

• Pointer Events (pointerType) for Android styluses and pencil hover, if wanted.

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