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/commands/variableCommands.ts

@@ -2,7 +2,7 @@ import { nodeDef } from '../graph/NodeRegistry';
 import type { BehaviorAsset } from '../model/Asset';
 import type { PropertyValue } from '../model/Properties';
 import { findAsset, findUsages, type Project } from '../model/Project';
-import { type PropertyDef, type VariableKind, variableDef } from '../properties/PropertySchema';
+import { isCommonProp, type PropertyDef, type VariableKind, variableDef } from '../properties/PropertySchema';
 import { type Command, CommandError } from './Command';
 
 export function requireBehavior(project: Project, assetId: string): BehaviorAsset {
@@ -128,5 +128,6 @@ export class RenameVariableCommand implements Command {
 }
 
 export function isValidVariableName(name: string): boolean {
-  return /^[A-Za-z_][A-Za-z0-9_]*$/.test(name);
+  // Common property keys (enabled) are reserved: a variable of that name would shadow the switch.
+  return /^[A-Za-z_][A-Za-z0-9_]*$/.test(name) && !isCommonProp(name);
 }