refactor: move world concepts into engine

game/actors/Boulder.ts

@@ -1,5 +1,7 @@
 import type { Vec3 } from "../../engine/math/Vec3"
 import { STRIDE, type Mesh } from "../../engine/scene/Mesh"
+import type { Material } from "../../engine/render/Material"
+import type { Prefab } from "../../engine/scene/Prefab"
 
 const TAU = Math.PI * 2
 
@@ -22,8 +24,35 @@ export type Boulder = {
  * field of boulders batches into a single draw call.
  */
 export namespace Boulder {
+  export function create(material: Material): Prefab<Boulder> {
+    return {
+      position: (boulder) => boulder.position,
+      bakeNear: (boulder, batch) => build(boulder, batch.mesh(material)),
+      bakeFar: (boulder, batch) =>
+        build(boulder, batch.mesh(material), "impostor"),
+      collider(boulder) {
+        if (boulder.radius <= 0.7) {
+          return null
+        }
+        return {
+          shape: "box",
+          minX: boulder.position.x - boulder.radius,
+          maxX: boulder.position.x + boulder.radius,
+          minZ: boulder.position.z - boulder.radius,
+          maxZ: boulder.position.z + boulder.radius,
+          top: boulder.position.y + boulder.radius * 0.7,
+          standable: false,
+        }
+      },
+    }
+  }
+
   /** `lod` "impostor" bakes a coarser rock (fewer facets) for far chunks. */
-  export function build(boulder: Boulder, mesh: Mesh, lod: "full" | "impostor" = "full"): void {
+  export function build(
+    boulder: Boulder,
+    mesh: Mesh,
+    lod: "full" | "impostor" = "full",
+  ): void {
     const rand = rng(boulder.seed)
     const seg = lod === "impostor" ? 4 : 5
     const rings = lod === "impostor" ? 2 : 4
@@ -67,7 +96,11 @@ export namespace Boulder {
   /** Per-vertex radial scale in ~0.72..1.14 for a chunky, angular surface. The
    *  longitude seam (last column == first) and each pole row (one shared value)
    *  match so the mesh stays closed. */
-  function jitterGrid(seg: number, rings: number, rand: () => number): number[][] {
+  function jitterGrid(
+    seg: number,
+    rings: number,
+    rand: () => number,
+  ): number[][] {
     const grid: number[][] = []
     for (let ir = 0; ir <= rings; ir++) {
       const pole = ir === 0 || ir === rings