M7: Engine adapters and export

[TristinWen]

Ordered scope

1. Finish Unity native Prefab compilation and asset metadata
2. Adapter conformance suite
3. Godot scene adapter
4. Unreal Actor adapter
5. glTF export
6. FBX/USD export where toolchains and licensing permit

Begin only after Core contracts and conformance requirements stabilize.

[TristinWen]

Partial implementation in eb652b2 and d431b7b: Unity now compiles self-describing Prefabs with persistent meshes/source fingerprints, and Core provides an adapter conformance suite. Godot, Unreal, geometry-preserving glTF, and optional licensed FBX/USD work remain; this issue stays open.