Add shared-scene 2D authoring, sprite animation and Rapier2D physics

docs/ARCHITECTURE.md

@@ -8,6 +8,10 @@ The editor and MCP mutate one serializable project through `ProjectStore`. The r
 | `engine/store.ts` | Atomic transactions, revisions, undo/redo and request receipts |
 | `engine/geometry.ts` | Procedural geometry and compound models |
 | `engine/runtime.ts` | Babylon rendering, model loading, animation, physics, input and worker bridge |
+| `engine/two-d.ts`, `engine/image-import.ts` | 2D document validation, frames, tile tools and image import |
+| `engine/graphics2d.ts`, `engine/view2d.ts` | Shared-scene sprites, animation, batched tiles and XY editor controls |
+| `engine/physics2d.ts` | Independent Rapier2D world, controller, sensors, collision filters and joints |
+| `engine/template2d.ts` | Empty XY project and original runnable platformer example |
 | `engine/character.ts` | Fixed-step Rapier character movement and contacts |
 | `engine/script-worker.js` | Project behaviour lifecycle and command output |
 | `engine/templates.ts` | Blank project construction |
@@ -21,7 +25,7 @@ The editor and MCP mutate one serializable project through `ProjectStore`. The r
 
 ## Document and runtime
 
-Transforms are local to an entity's parent. Reparenting preserves local coordinates unless a transform is supplied. Duplicate and prefab operations remap internal parent, camera target and typed entity-property references; external references remain unchanged.
+Transforms are local to an entity's parent. Reparenting preserves local coordinates unless a transform is supplied. Duplicate and prefab operations remap internal parent, camera target, 2D joint target and typed entity-property references; external references remain unchanged.
 
 Transactions apply atomically. A stale `expectedRevision` fails instead of overwriting intervening edits. Repeating a transaction with the same `requestId` can reuse the recorded result. History and receipts are held in memory, while the project document is persisted.