Rapier 2D (@dimforge/rapier2d-compat 0.20) comes in as the physics engine: the editor loads it once in the background right after opening a project from its own chunk, so the main bundle grows by 0.19 KB gzip and the first frame does not wait; loadPhysics, physicsStatus and the 100 px = 1 m conversion live in src/runtime/physics/engine.ts, __gm.physics() reports the state, unit tests pin free fall and a box resting on a box, and e2e module 41 pins that Rapier is ready from its own chunk without console noise (#145)

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

src/runtime/physics/engine.ts

@@ -0,0 +1,40 @@
+import type RAPIER from '@dimforge/rapier2d-compat';
+
+export type Rapier = typeof RAPIER;
+
+/** Everything outside the engine is in pixels; Rapier is tuned for metres. */
+export const PIXELS_PER_METRE = 100;
+
+export const toMetres = (pixels: number): number => pixels / PIXELS_PER_METRE;
+export const toPixels = (metres: number): number => metres * PIXELS_PER_METRE;
+
+export type PhysicsStatus =
+  | { state: 'idle' | 'loading' }
+  | { state: 'ready'; ms: number; version: string }
+  | { state: 'failed'; error: string };
+
+let status: PhysicsStatus = { state: 'idle' };
+let loading: Promise<Rapier> | null = null;
+
+/**
+ * Rapier is its own chunk with the WebAssembly inside. Every caller shares the one load, and a
+ * failed load stays failed rather than being tried again on each call.
+ */
+export function loadPhysics(): Promise<Rapier> {
+  if (loading) return loading;
+  const start = performance.now();
+  status = { state: 'loading' };
+  loading = import('@dimforge/rapier2d-compat').then(async ({ default: rapier }) => {
+    await rapier.init();
+    status = { state: 'ready', ms: Math.round(performance.now() - start), version: rapier.version() };
+    return rapier;
+  });
+  loading.catch((error: unknown) => {
+    status = { state: 'failed', error: error instanceof Error ? error.message : String(error) };
+  });
+  return loading;
+}
+
+export function physicsStatus(): PhysicsStatus {
+  return status;
+}