wmo: apply MOHD ambient color to interior group lighting

Read the ambient color from the MOHD chunk (BGRA uint32) and store it
on WMOModel as a normalized RGB vec3.  Pass it through ModelData into
the per-batch WMOMaterialUBO (replacing the unused pad[3] bytes, keeping
the struct at 64 bytes).  The GLSL interior branch now floors vertex
colors against the WMO ambient instead of a hardcoded 0.5, so dungeon
interiors respect the artist-specified ambient tint from the WMO root
rather than always clamping to grey.

include/pipeline/wmo_loader.hpp

@@ -185,6 +185,7 @@ struct WMOModel {
     uint32_t nDoodadDefs;
     uint32_t nDoodadSets;
 
+    glm::vec3 ambientColor;     // MOHD ambient color (used for interior group lighting)
     glm::vec3 boundingBoxMin;
     glm::vec3 boundingBoxMax;