Initial commit: Game Maker phase 1

Canvas-only 2D game editor/runtime built on PixiJS v8 + Vite + TypeScript.

- core: asset model (Scene/Camera/Sprite), scene objects with sparse
  overrides, property schema + resolve, command stack (undo/redo),
  serialization and project store interface (16 unit tests)
- ui: custom in-canvas widget toolkit (panels, fields, color picker,
  list/scroll, popup) with a hidden <input> keyboard bridge
- editor: asset/hierarchy panels, schema-driven inspector, toolbar
- viewport: box rendering, camera frame, selection gizmo, drag/zoom/pan
- runtime: world/entity instantiation for play mode, input, camera follow
- api: PHP JSON project store with revision counter (409 on conflict)
- tools: CDP-based e2e smoke test

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

src/core/store/ProjectStore.ts

@@ -0,0 +1,31 @@
+import type { Project } from '../model/Project';
+
+export interface ProjectSummary {
+  id: string;
+  name: string;
+  revision: number;
+  updatedAt: string;
+}
+
+export interface LoadedProject {
+  project: Project;
+  revision: number;
+}
+
+/** Thrown by `save` when the server holds a newer revision (edited elsewhere). */
+export class StaleRevisionError extends Error {
+  constructor(readonly serverRevision: number) {
+    super(`Project was modified elsewhere (server revision ${serverRevision})`);
+    this.name = 'StaleRevisionError';
+  }
+}
+
+/** Persistence boundary. The editor only talks to this interface. */
+export interface ProjectStore {
+  list(): Promise<ProjectSummary[]>;
+  load(id: string): Promise<LoadedProject>;
+  /** Saves the project and returns the new revision. Rejects with StaleRevisionError on conflict. */
+  save(project: Project, revision: number): Promise<number>;
+  create(name: string): Promise<LoadedProject>;
+  delete(id: string): Promise<void>;
+}