serialization: scene save/load (Unity-style YAML + UE-style GUID registry)

Engine-layer SceneSerializer (yaml-cpp submodule): entities/components
to versioned YAML, atomic clear-and-rebuild on load with per-entity
fault tolerance, missing-field defaults, unknown-component skip and
UUID dedup. Asset references are {Guid, Path} pairs resolved through
the new AssetManager GUID registry (assets/asset_registry.yaml) with
path fallback; procedural:// pseudo-paths rebuild from parameters.
Editor File menu gets Save/Load Scene modals; app re-resolves stale
entity handles via onSceneLoaded callback.

Engine/CMakeLists.txt

@@ -73,6 +73,7 @@ target_link_libraries(Engine PUBLIC
         assimp::assimp
         meshoptimizer
         Jolt::Jolt
+        yaml-cpp::yaml-cpp
         imgui
         ImGuizmo
         tracy)