Enabled switch, text assets, mouse input and the jump-and-run example game

Engine additions that the example game needed:
- `enabled` as a shared property of every placeable asset (default on, an
  override on the scene object). A switched-off object is not drawn, does not
  collide and its behavior does not run - and neither does anything parented
  below it. Disabled objects are now hidden in the scene view as well, so the
  editor shows what the game shows; they stay in the hierarchy panel, which is
  where they are selected and switched back on.
- Asset type `text` (text, fontSize, color, align), rendered in scene and game.
- Pointer input in the game view: `On click`, `Mouse position`, `Mouse button`,
  `Is hovered`, hit-tested against colliders, else sprite/text footprints.
- New nodes: Set/Is enabled, Set text, Is asset, Name of, Equals, To text,
  Join text, Floor/Ceil/Round, Modulo, Random, Time, Restart game.

Tooling: shared CDP harness (tools/harness.mjs), central project registry with
a guard that keeps destructive scripts off the playground (tools/projects.mjs),
plus smoke suites for the demo scene and the jump-and-run game.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/store/serialize.ts

@@ -5,7 +5,7 @@ import { isAssetType } from '../model/AssetTypes';
 import type { PropertyBag, PropertyValue } from '../model/Properties';
 import { PROJECT_FORMAT_VERSION, type Project } from '../model/Project';
 import type { SceneObject } from '../model/SceneObject';
-import { coerceValue, type PropertyDef, type PropertySchema, schemaFor, VARIABLE_KINDS, variableDef, type VariableKind } from '../properties/PropertySchema';
+import { coerceValue, COMMON_PROPS, isCommonProp, type PropertyDef, type PropertySchema, schemaFor, VARIABLE_KINDS, variableDef, type VariableKind } from '../properties/PropertySchema';
 
 /**
  * Turns untrusted JSON (server file, import) into a well-formed Project. Unknown keys are
@@ -25,7 +25,7 @@ export function deserializeProject(raw: unknown, fallbackId = ''): Project {
     if (a.type === 'behavior') {
       const vars = normalizeVariables(a.variables);
       variables.set(a.id, vars);
-      schemas.set(a.id, vars);
+      schemas.set(a.id, [...COMMON_PROPS, ...vars]);
     } else {
       schemas.set(a.id, schemaFor(a.type));
     }
@@ -113,7 +113,8 @@ function normalizeVariables(raw: unknown): PropertyDef[] {
     const v = asRecord(item);
     const key = typeof v.key === 'string' ? v.key : '';
     const kind = (VARIABLE_KINDS as readonly string[]).includes(String(v.kind)) ? (v.kind as VariableKind) : 'number';
-    if (!/^[A-Za-z_][A-Za-z0-9_]*$/.test(key) || seen.has(key)) continue;
+    // Common property keys are reserved – a variable of that name would shadow the switch.
+    if (!/^[A-Za-z_][A-Za-z0-9_]*$/.test(key) || seen.has(key) || isCommonProp(key)) continue;
     seen.add(key);
     const def = variableDef(key, kind);
     def.default = coerceValue(def, (v.default ?? def.default) as PropertyValue) as never;