Let the project set the size of scene view symbols

Pixel art projects draw at very different sizes, so the fixed 18 px
icons of behaviors, containers and rigidbodies were too big in some
projects and too small in others. The size now lives in the project
settings and travels with the project.

- New field Symbol scale in the project settings dialog, below Play
  starts. It takes 0.1 to 4 with two decimals and defaults to 1.
- The value is stored as project.symbolScale. Older projects load
  with 1, and values out of range are clamped on load.
- Every change is its own undo step through a project command.
- The factor scales the icon size, the spacing of icons that share a
  spot, the label offset and the selection frame of icon-only symbols.
- The footprints in bounds.ts that Fit uses stay as they were. The
  hit area follows the icon, so clicks match what is drawn.
- uiHooks reports the icon size so e2e can measure it.

The e2e sum in the README is 1375. That is 1374 plus one new check,
and 1374 itself has not been confirmed by CI yet.

Tests:
- ./check.sh: 157 files, 2326 tests, CHECK EXIT 0 (before 2320).
- New tests/core/symbol-scale.test.ts with 5 tests (default, save
  and load, clamping, undo, icon frame), one more in
  project-settings.test.ts.
- e2e 46f: dialog opens with 1, 0.5 plus Apply gives a 9 px icon,
  undo gives 18 again. Module 46: ALL PASSED, 13 checks.
- Counter checks: without loading the value the save and load test
  fails. With the icon ignoring the factor exactly 46f fails. Both
  originals restored per cmp.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/e2e/46-topbar.mjs

@@ -133,5 +133,38 @@ export async function topbar(t) {
     (await json('__gm.state.project.name')) === projectName,
     await json('__gm.state.project.name'),
   );
+
+  // 46f. The symbol scale in the same dialog: 1 by default, typed down to 0.5, and every icon of the
+  //      scene drawn at half its side (18 → 9 world units), undone in one step.
+  const iconSize = () =>
+    json(`(() => {
+      const p = __gm.state.project;
+      const scene = p.assets.find((a) => a.id === p.activeSceneId);
+      const id = (scene?.objects ?? []).map((o) => o.id).find((id) => __gm.sceneIcon(id)?.visible);
+      return id ? __gm.sceneIcon(id).size : null;
+    })()`);
+  await clickToolbar('menu');
+  await clickText('Project settings…');
+  const opened = await json('__gm.projectSettings()');
+  const field = await json('__gm.projectSettingsSymbolScaleAt()');
+  await t.click(field.x, field.y);
+  await press('a', { ctrl: true });
+  await typeText('0.5');
+  await press('Enter');
+  await clickText('Apply');
+  await sleep(200);
+  const scaled = { project: await json('__gm.state.project.symbolScale'), icon: await iconSize() };
+  await clickToolbar('undo');
+  await sleep(200);
+  const restored = { project: await json('__gm.state.project.symbolScale'), icon: await iconSize() };
+  check(
+    'the symbol scale starts at 1, Apply halves every scene icon, and undo gives the size back',
+    opened?.symbolScale === 1 &&
+      scaled.project === 0.5 &&
+      Math.abs(scaled.icon - 9) < 1e-9 &&
+      restored.project === 1 &&
+      Math.abs(restored.icon - 18) < 1e-9,
+    JSON.stringify({ opened, scaled, restored }),
+  );
   await t.saved();
 }