refactor: move world concepts into engine

engine/scene/Prefab.ts

@@ -0,0 +1,38 @@
+import type { Vec3 } from "../math/Vec3"
+import type { MeshBatch } from "../render/ChunkBuilder"
+import type { Collider } from "../world/Collider"
+
+/** Static content recipe. Concrete game prefabs implement this engine contract and
+ * are referenced as objects, never through a closed content-kind registry. */
+export type Prefab<State> = {
+  position: (state: State) => Vec3
+  bakeNear: (state: State, batch: MeshBatch) => void
+  bakeFar?: (state: State, batch: MeshBatch) => void
+  collider?: (state: State) => Collider | null
+}
+
+/** Type-erased placed prefab consumed during level compilation only. */
+export type PlacedPrefab = {
+  position: Vec3
+  bakeNear: (batch: MeshBatch) => void
+  bakeFar?: (batch: MeshBatch) => void
+  collider: Collider | null
+}
+
+export namespace Prefab {
+  export function place<State>(
+    definition: Prefab<State>,
+    state: State,
+  ): PlacedPrefab {
+    const bakeFar = definition.bakeFar
+    return {
+      position: definition.position(state),
+      bakeNear: (batch) => definition.bakeNear(state, batch),
+      bakeFar:
+        bakeFar === undefined
+          ? undefined
+          : (batch) => bakeFar(state, batch),
+      collider: definition.collider?.(state) ?? null,
+    }
+  }
+}