Round number fields to the precision they display

`decimals` only reached `formatNumber`, so it changed the label and nothing else: typing 640.5 into
a field with `decimals: 0` stored 640.5 and displayed 641. The two existing users got away with it
because rounding happened outside the widget (`normalizeFrameRect`, the slice dialog), which their
comments had to point out.

`NumberField.clampValue` now rounds before it clamps, and every path that sets a value goes through
it (typing, scrubbing, arrow keys, the constructor), so the stored number matches the shown one and
`min`/`max` stay exact. The rounding is `roundTo` in math.ts, which `snapToStep` already did inline.

With that safe, camera `View width`/`View height` and text `Font size` become integers – they were
showing 640.000 and 32.000. Everything else that looked integral is not: positions, sprite and
collider sizes and zoom are all scrubbed, and `z` is a summed coordinate (walk.ts), not a layer.

Verified remotely against the RTX 4070 host: 270 checks, all passed (step 10 gained a check that the
field text has no decimal point and survives a reopen). `npm run check` green, 407 tests.

src/editor/texture/SliceDialog.ts

@@ -84,7 +84,6 @@ export class SliceDialog extends Popup {
     const field = new NumberField(this.options[key], {
       min,
       step: 1,
-      // Counts and pixel offsets, rounded on change, so 0 decimals hides nothing.
       decimals: 0,
       onChange: (value) => {
         this.options[key] = Math.round(Number(value));