refactor: move world concepts into engine

CONTEXT.md

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+# Meat Engine
+
+Meat separates reusable first-person game capabilities from one game's concrete world and creatures.
+
+## Language
+
+**Level Definition**:
+A complete engine-shaped input for creating one live Level. Game code produces it from concrete values and definitions; it is not the worker rendering payload.
+_Avoid_: Scene assembly
+
+**Level**:
+A live playable world created from a Level Definition, including terrain, actors, collision, and rendering state.
+_Avoid_: Map, scene
+
+**Terrain**:
+A bounded ground surface that can report height at any horizontal world position.
+_Avoid_: Ground mesh
+
+**Actor Definition**:
+A concrete reusable definition of one moving thing's behavior, representation, transform, and physical presence. Actor instances reference definitions directly.
+_Avoid_: Mob kind, actor kind, type registry
+
+**Prefab**:
+A concrete reusable definition for static level content, including its geometry and optional physical presence. Placed prefabs reference definitions directly.
+_Avoid_: Prop kind, tree kind, species registry
+
+**Render Scene**:
+The behavior-free rendering projection of a Level.
+_Avoid_: Level