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/core/properties/PropertySchema.ts

@@ -5,8 +5,9 @@ import type { PropertyBag, PropertyValue } from '../model/Properties';
 /** The inspector is generated from these entries, so a new property is one line in a schema below. */
 export type PropertyDef =
   /**
-   * `decimals` belongs only on properties that are integral by meaning (counts, indices), not on
-   * geometry: dragging produces fractions, and a field that hid them would commit the rounded value.
+   * `decimals` belongs only on properties that are integral by meaning (counts, sizes in whole
+   * pixels), not on geometry: dragging produces fractions, and the field rounds the value away
+   * rather than hiding it.
    */
   | {
       key: string;
@@ -58,8 +59,9 @@ export function isCommonProp(key: string): boolean {
 const ASSET_SCHEMAS: Record<AssetType, PropertySchema> = {
   scene: [{ key: 'backgroundColor', label: 'Background', kind: 'color', default: 0x2a2d34 }],
   camera: [
-    { key: 'width', label: 'View width', kind: 'number', default: 640, min: 16, step: 1 },
-    { key: 'height', label: 'View height', kind: 'number', default: 360, min: 16, step: 1 },
+    // A view is a pixel raster, so half a pixel of it means nothing.
+    { key: 'width', label: 'View width', kind: 'number', default: 640, min: 16, step: 1, decimals: 0 },
+    { key: 'height', label: 'View height', kind: 'number', default: 360, min: 16, step: 1, decimals: 0 },
     { key: 'zoom', label: 'Zoom', kind: 'number', default: 1, min: 0.1, max: 10, step: 0.1 },
   ],
   sprite: [
@@ -104,7 +106,7 @@ const ASSET_SCHEMAS: Record<AssetType, PropertySchema> = {
   container: [],
   text: [
     { key: 'text', label: 'Text', kind: 'text', default: 'Text' },
-    { key: 'fontSize', label: 'Font size', kind: 'number', default: 32, min: 1, step: 1 },
+    { key: 'fontSize', label: 'Font size', kind: 'number', default: 32, min: 1, step: 1, decimals: 0 },
     { key: 'color', label: 'Color', kind: 'color', default: 0xffffff },
     {
       key: 'align',