fix(core): keep Havok physics out of bundles when features.physics is disabled

Summary:
\`create-iwsdk\` defaults to \`--no-physics\` and most scaffolded apps never enable physics, but every IWSDK build was shipping the full Havok physics engine: a separate Havok JS chunk **plus the ~2 MB \`HavokPhysics.wasm\` binary**, plus a stream of physics symbols (\`PhysicsBody\`/\`PhysicsSystem\`/\`HP_World\`/\`MotionType\`) in the main bundle. Verified on \`examples/poke\` (\`features.physics: false\` set explicitly), pre-fix: main bundle 3.0 MB / 791 KB gzip + 2.0 MB WASM + 36 KB Havok-es chunk — all downloaded on first load.

Two static pins were keeping \`babylonjs/havok\` and the physics module in the static module graph regardless of \`features.physics\`:

1. **\`packages/core/src/init/world-initializer.ts\`** statically imported \`{ PhysicsBody, PhysicsManipulation, PhysicsShape, PhysicsSystem }\` from \`../physics/index.js\` and only *used* them inside \`if (physicsEnabled)\`. The import alone was enough to pull the physics graph in.
2. **\`packages/core/src/physics/physics-system.ts\`** statically imported \`{ MotionType, ... }\` from \`babylonjs/havok\`. \`HP_ShapeId\`/\`HP_WorldId\`/\`MassProperties\`/\`HavokPhysicsWithBindings\` are TS type-only (erased), but \`MotionType\` is a runtime \`enum\` — so \`babylonjs/havok\` was a real runtime dep on this code path. The actual engine load on line 87 (\`await import('babylonjs/havok')\`) was already dynamic and would have given proper code-splitting on its own.

And one barrel re-export was blocking tree-shaking: \`iwsdk/core/package.json\` had no \`sideEffects\` declaration, so every module was assumed to have side effects and Rollup couldn't drop unused exports re-exported via \`export * from './physics/index.js'\` in the core barrel.

This change:

- Converts the world-initializer physics imports to \`await import('../physics/index.js')\` inside the \`physicsEnabled\` branch. \`registerFeatureSystems\` and \`initializeWorld\` become \`async\` accordingly; \`initializeWorld\` already returned \`Promise<World>\` so the externally-visible signature is unchanged.
- Switches \`physics-system.ts\`'s havok imports to \`import type { ... }\`. \`MotionType\` is the only one that was a runtime symbol, and the file already accesses it at runtime through the loaded instance (\`this.havok.MotionType.STATIC\` etc.) — the static import was only providing the type annotation on \`let motionType: MotionType\` (line 307), which is compile-erased.
- Adds \`"sideEffects": ["dist/index.js"]\` to \`iwsdk/core/package.json\`. The barrel's banner \`console.log\` (the IWSDK ASCII art) is the only intentional top-level side effect; every other module is \`createComponent\`/\`createSystem\` const-binding + class definitions with no module-load side effects. Marking just the barrel as having side effects lets Rollup tree-shake unused exports — including everything physics if no consumer references a physics symbol.

After the fix on \`examples/poke\`:
- Entry HTML loads only the main bundle (2.9 MB) — no physics code, no Havok strings.
- The physics module becomes a separate 16 KB chunk that is *generated* but never *fetched* unless \`features.physics\` is true.
- Havok-es chunk (36 KB) and \`HavokPhysics.wasm\` (2 MB) are still emitted as cold assets the physics chunk would import on-demand, but no consumer fetches them.
- Browser-downloaded bytes for a \`physics: false\` page drop from ~5 MB to 2.9 MB (~2.1 MB saved on first load).

\`examples/physics\` (\`features.physics: true\`): main bundle still 2.9 MB; physics chunk loads on demand; Havok WASM still fetched as before. No regression.

Reviewed By: zjm-meta

Differential Revision: D105972540

fbshipit-source-id: 7c532b19469cc0344d859bda1fe258df8192d58b

Author: felixtrz

packages/core/package.json

@@ -8,6 +8,29 @@
   "files": [
     "dist"
   ],
+  "sideEffects": [
+    "dist/index.js",
+    "dist/asset/**",
+    "dist/audio/**",
+    "dist/camera/**",
+    "dist/depth/**",
+    "dist/ecs/**",
+    "dist/environment/**",
+    "dist/environment-raycast/**",
+    "dist/grab/**",
+    "dist/init/**",
+    "dist/input/**",
+    "dist/layers/**",
+    "dist/level/**",
+    "dist/locomotion/**",
+    "dist/mcp/**",
+    "dist/runtime/**",
+    "dist/scene-understanding/**",
+    "dist/transform/**",
+    "dist/ui/**",
+    "dist/vendor/**",
+    "dist/visibility/**"
+  ],
   "scripts": {
     "prebuild": "node -p \"'export const VERSION = ' + JSON.stringify(require('./package.json').version) + ';'\" > src/version.ts",
     "build": "rollup -c",

packages/core/src/init/world-initializer.ts

@@ -47,12 +47,10 @@ import {
   TurningMethod,
 } from '../locomotion/index.js';
 import { MCPRuntime } from '../mcp/index.js';
-import {
-  PhysicsBody,
-  PhysicsManipulation,
-  PhysicsShape,
-  PhysicsSystem,
-} from '../physics/index.js';
+// The physics module pulls in `@babylonjs/havok` (~2 MB WASM + engine JS) at
+// the static-graph level. Load it lazily in `registerFeatureSystems` only
+// when `features.physics` is enabled so non-physics projects don't pay the
+// bundle-size tax.
 import {
   Clock,
   PerspectiveCamera,
@@ -200,7 +198,7 @@ type CameraPoseOptions = NonNullable<
  * @remarks
  * This function powers {@link World.create}. Prefer using that static helper.
  */
-export function initializeWorld(
+export async function initializeWorld(
   container: HTMLDivElement,
   options: WorldOptions = {},
 ): Promise<World> {
@@ -234,8 +232,10 @@ export function initializeWorld(
   // Register additional systems (UI + Audio on by default)
   registerAdditionalSystems(world);
 
-  // Register input and feature systems with explicit priorities
-  registerFeatureSystems(world, config);
+  // Register input and feature systems with explicit priorities.
+  // Awaited because the physics branch dynamically imports its module so
+  // `@babylonjs/havok` stays out of the static graph for non-physics builds.
+  await registerFeatureSystems(world, config);
 
   // Setup render loop
   setupRenderLoop(world, renderer);
@@ -594,7 +594,7 @@ function registerAdditionalSystems(world: World) {
   world.registerComponent(AudioSource).registerSystem(AudioSystem);
 }
 
-function registerFeatureSystems(
+async function registerFeatureSystems(
   world: World,
   config: ReturnType<typeof extractConfiguration>,
 ) {
@@ -673,8 +673,12 @@ function registerFeatureSystems(
     world.registerSystem(GrabSystem, { priority: -3, configData: grabOpts });
   }
 
-  // Physics runs after Grab so it can respect Pressed overrides
+  // Physics runs after Grab so it can respect Pressed overrides.
+  // Dynamically imported so the physics module (and its ~2 MB Havok WASM)
+  // stays out of the static module graph for non-physics projects.
   if (physicsEnabled) {
+    const { PhysicsBody, PhysicsManipulation, PhysicsShape, PhysicsSystem } =
+      await import('../physics/index.js');
     world
       .registerComponent(PhysicsBody)
       .registerComponent(PhysicsShape)

packages/core/src/physics/physics-system.ts

@@ -5,7 +5,14 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-import {
+// All symbols from `@babylonjs/havok` are type-only here. The runtime engine
+// (which ships ~2 MB of WASM) is loaded lazily via `await import(...)` later
+// in `init()`, and runtime access to enums like `MotionType` is done via the
+// loaded instance (e.g. `this.havok.MotionType.STATIC`). Keeping these as
+// `import type` ensures `@babylonjs/havok` stays out of the static module
+// graph when consumers don't enable `features.physics`, so the WASM and JS
+// engine chunks aren't bundled into non-physics projects.
+import type {
   HavokPhysicsWithBindings,
   HP_ShapeId,
   HP_WorldId,