feat: actors stage 2

engine/scene/Actor.ts

@@ -0,0 +1,26 @@
+import type { Mesh } from "./Mesh"
+
+/** Definition of an **Entity** actor kind: something that lives in the world with
+ *  its own behavior and a live transform (mobs, and later the npc / powerups) -- as
+ *  opposed to a baked, static `Prop`. `State` is the per-instance runtime record the
+ *  behavior mutates; `World` is whatever that behavior reads (e.g. `Terrain`).
+ *
+ *  Every field is plain data or a module function, so a definition is **imported
+ *  into each context** (main thread + each render worker) rather than structured-
+ *  cloned across the wire -- the per-kind polymorphism is code, not serialized
+ *  state. That's what lets a registry of these stay compatible with the worker
+ *  renderer (only plain instance data ever crosses; behavior is loaded per side). */
+export type Entity<State, World> = {
+  /** Stable tag for the kind (also the texture key today). The registry's order,
+   *  not this string, is what becomes the id packed into the mob SAB. */
+  name: string
+  /** Build the canonical local-space mesh once; every instance shares it, differing
+   *  only by its per-frame model matrix. */
+  build: (mesh: Mesh) => void
+  /** Advance one instance by `dt` seconds. */
+  update: (state: State, dt: number, world: World) => void
+  /** Local bounding radius (pre-scale) for the per-frame cull AABB. */
+  boundingRadius: number
+  /** Local body height (pre-scale) for the top of the stand-on collider. */
+  bodyHeight: number
+}