web: an Apple Pencil counts as touch; treat a stylus as a pen #568

Follow-up from #563, the pointer seam (dvui.pointerKind()).

On the native backend, SDL's pen and mouse touch ids are excluded, so a pen reads as .mouse. On the web, web.js turns every TouchEvent into a dvui touch button. An Apple Pencil on iPad Safari sends TouchEvents with touch.touchType === "stylus", so a pencil counts as .touch there. That's consistent, not flipping, but pixi users drawing with a pencil on the web get touch behaviour: no hover after a lift, and touch-style panning in canvases.

Fix in web.js (dvui's web glue, pinned from dvui-dev):

• Route touchType === "stylus" touches as mouse/pen input rather than touch buttons.
• Or move to Pointer Events and use pointerType ("pen" / "touch" / "mouse"), which also covers Android styluses.

That would make a separate PR, with a dvui-dev commit and a repin.