PBR maps and RenderSettings

Material maps:
- normal maps (with _BumpScale / glTF's scale), in a tangent frame computed per mesh on upload with Unity's handedness;
- metallic maps (Standard's and URP's R metallic, A smoothness times the scale; glTF's B metallic, G roughness times the factors);
- occlusion (Standard's G with _OcclusionStrength; glTF's R with its strength) darkening ambient light and reflections;
- emission maps multiplying the emission color.
Maps apply behind Unity's keywords (_NORMALMAP, _METALLICGLOSSMAP, URP's _METALLICSPECGLOSSMAP and _OCCLUSIONMAP, _EMISSION). .mat files now import their keywords.
glTF imports every map with glTFast's factors and keywords, color images in sRGB and the others linear, embedded and linear images without mipmaps (as glTFast does).
Texture .meta files: normal maps and sRGB-off textures load linear, and Texture.isDataSRGB reports it.
RenderSettings:
- each scene has its own (Unity's defaults for a new one); loading a .unity scene brings its RenderSettings and sun;
- making another scene active switches to its settings; without a sun, sun is the brightest directional light;
- JSON scenes take a "renderSettings" object, which the Unity round trip uses.
The renderer lights from the trilight ambient (or flat, or the skybox stand-in times the intensity), scales reflections by reflectionIntensity, and applies linear, exponential and squared fog by view depth. Sprite and UI shaders skip the fog.
MCP inline materials take "textures" (maps turn their keywords on), "colors" and "keywords"; textures take "linear".
The conformance project now uses Linear color space, as Tassix renders.
Conformance:
- a new render_settings scenario records defaults, script changes, a Unity scene's settings and switching the active scene;
- gltf_import gained a generated model with every map (conformance/assets/make_pbr_maps.py);
- 36 scenarios pass in both runs.
GPU tests cover normal, metallic and occlusion maps in both channel layouts, trilight ambient and fog. MCP e2e tests cover fog and ambient from a script, and an inline emission map.
Not yet: the MCP editor keeping a scene's RenderSettings, so playing a .unity scene from MCP uses the defaults; skyboxes and reflection probes; detail and height maps; Mesh.tangents.
Next: the MCP editor's RenderSettings, then skyboxes (procedural and cubemap) as backgrounds and ambient.