Camera view frame, viewport clipping, overlay layer for editor symbols

- Camera assets get width/height properties; play mode renders only the
  camera frame (letterboxed, masked, culled) and nothing without a camera
- Remove demo arrow-key/camera-follow systems from play mode
- Clip world and gizmos to the viewport (selection frame no longer leaks
  over the left panel)
- Blur focused widget on pointerdown outside it so inspector edits commit
  before selection changes
- Separate objects/overlay layers so camera symbols always render on top
- Add tools/seed-demo.mjs for reproducible E2E runs

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/core/properties/PropertySchema.ts

@@ -18,7 +18,11 @@ export type PropertySchema = readonly PropertyDef[];
 /** Property schema per asset type. Position is deliberately absent: it belongs to scene objects. */
 export const ASSET_SCHEMAS: Record<AssetType, PropertySchema> = {
   scene: [{ key: 'backgroundColor', label: 'Background', kind: 'color', default: 0x2a2d34 }],
-  camera: [{ key: 'zoom', label: 'Zoom', kind: 'number', default: 1, min: 0.1, max: 10, step: 0.1 }],
+  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 },
+    { key: 'zoom', label: 'Zoom', kind: 'number', default: 1, min: 0.1, max: 10, step: 0.1 },
+  ],
   sprite: [
     { key: 'width', label: 'Width', kind: 'number', default: 100, min: 1, step: 1 },
     { key: 'height', label: 'Height', kind: 'number', default: 100, min: 1, step: 1 },