feat: actors stage 2

tests/mobs.test.ts

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+import { expect, test } from "bun:test"
+import { MOB_KINDS, Mob } from "../engine/scene/Mob"
+
+// The mob SAB packs a kind as its index in MOB_KINDS; the main thread and every
+// render worker must agree on that order. Freeze it here: appending a kind is fine,
+// but reordering or removing an existing one silently corrupts which mesh/texture a
+// worker draws.
+test("MOB_KINDS order is frozen (mob SAB ids)", () => {
+  expect(MOB_KINDS).toEqual(["frog", "bee", "robin"])
+})
+
+test("every kind resolves to a complete definition", () => {
+  for (const kind of MOB_KINDS) {
+    const d = Mob.def(kind)
+    expect(d.name).toBe(kind)
+    expect(typeof d.build).toBe("function")
+    expect(typeof d.update).toBe("function")
+    expect(d.boundingRadius).toBeGreaterThan(0)
+    expect(d.bodyHeight).toBeGreaterThan(0)
+  }
+})

tests/trees.test.ts

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+import { expect, test } from "bun:test"
+import { Tree, TREE_KINDS } from "../engine/scene/Tree"
+
+// The chunk baker (level.ts) accumulates geometry into a mesh per material key and
+// only draws keys listed in MAT_ORDER. A tree species that declares a trunk/foliage
+// material outside that palette would bake geometry that is silently never drawn.
+// Freeze the palette here so a typo'd or unregistered material key fails a test.
+const MATERIAL_KEYS = new Set(["grass", "rock", "bark", "birch", "leaf", "needle", "flower"])
+
+test("every tree species maps to known chunk materials", () => {
+  for (const kind of TREE_KINDS) {
+    const s = Tree.species(kind)
+    expect(s.kind).toBe(kind)
+    expect(typeof s.build).toBe("function")
+    expect(MATERIAL_KEYS.has(s.trunk)).toBe(true)
+    expect(MATERIAL_KEYS.has(s.foliage)).toBe(true)
+  }
+})