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.

tools/e2e/10-text-and-enabled.mjs

@@ -1,6 +1,21 @@
 /** Text assets rendered in the scene, and the enabled switch in the editor and in the running game. */
 export async function textAndEnabled(t) {
-  const { check, sleep, evaluate, json, shot, click, clickText, mutate, object, play, stop } = t;
+  const {
+    check,
+    sleep,
+    evaluate,
+    json,
+    shot,
+    click,
+    clickText,
+    locate,
+    mutate,
+    object,
+    play,
+    press,
+    stop,
+    typeText,
+  } = t;
   const has = (text) => evaluate(`Boolean(__gm.locate(${JSON.stringify(text)}))`);
 
   // 17. Text assets: created through the + menu, placed, and rendered as real text in the scene.
@@ -62,4 +77,16 @@ export async function textAndEnabled(t) {
   );
   await stop();
   check('stop leaves the play-mode switch behind', (await object('otext')).overrides.enabled === false);
+
+  // 17c. A field that shows no decimals must not store any either. Font size is such a field, so
+  //      typing a fraction has to round: showing "33" while the project holds 32.6 would be a lie.
+  const fontLabel = await locate('Font size');
+  check('Font size reads as a whole number', Boolean(await has('32')) && !(await has('32.000')));
+  await click(fontLabel.x + 160, fontLabel.y);
+  await typeText('32.6');
+  await press('Enter');
+  await sleep(200);
+  const fontSize = (await object('otext')).overrides.fontSize;
+  check('typing a fraction into it rounds the stored value', fontSize === 33, String(fontSize));
+  check('and the field shows exactly what was stored', Boolean(await has('33')));
 }