Wasm + iOS Safari: full of traps

I assumed wasm on mobile would Just Work. Yep, very naive of me.

The traps I tripped on:

• iOS Safari stack overflow during JSX eval. -O0 blew QuickJS's dispatch recursion past Apple's wasm stack limit. -O2 at link fixed it (and dropped the binary 13.5 MB -> 3.8 MB as a free bonus)
• Touch events did literally nothing. iOS doesn't reliably synthesize mousedown from a touch, had to wire touchstart myself
• Taps landed on the wrong button. targetX is CSS pixels, hit-test runs in canvas-buffer pixels, iOS shrinks the canvas via CSS. Multiply by canvas.width / canvas.clientWidth and we're good
• The default emscripten shell is REALLY ugly. Replaced with a 1.3 KB centered-canvas shell

Just Works, eventually (: