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.

tests/ui/math.test.ts

@@ -1,5 +1,13 @@
 import { describe, expect, it } from 'vitest';
-import { clamp, centreY, formatNumber, NUMBER_DECIMALS, snapToStep, zoomFactor } from '../../src/ui/math';
+import {
+  clamp,
+  centreY,
+  formatNumber,
+  NUMBER_DECIMALS,
+  roundTo,
+  snapToStep,
+  zoomFactor,
+} from '../../src/ui/math';
 
 describe('clamp', () => {
   it('keeps a value inside the range', () => {
@@ -60,6 +68,25 @@ describe('formatNumber', () => {
   });
 });
 
+describe('roundTo', () => {
+  it('keeps a value at the precision its field shows', () => {
+    // Why the field rounds instead of only formatting: a `decimals: 0` field used to display
+    // "641" while the project still held 640.5.
+    expect(roundTo(640.5, 0)).toBe(641);
+    expect(roundTo(32.4, 0)).toBe(32);
+    expect(roundTo(1.2345)).toBe(1.234);
+  });
+
+  it('removes binary-float noise', () => {
+    expect(roundTo(0.30000000000000004)).toBe(0.3);
+  });
+
+  it('passes non-finite values through', () => {
+    expect(roundTo(NaN)).toBeNaN();
+    expect(roundTo(Infinity)).toBe(Infinity);
+  });
+});
+
 describe('snapToStep', () => {
   it('lands on whole numbers for a step-1 field', () => {
     // The bug: a scrub added fractional device pixels to an already fractional value, so