Load OBJ material sidecars in importer

[Reza2kn]

Related

• Closes Loading obj files doesn't load associated mtl files #3773
• Follows up Fix loading of .obj mesh files #3772

What

This PR makes native OBJ file imports resolve their Wavefront material sidecars instead of logging the OBJ bytes as a standalone Asset3D blob.

When an .obj file is opened through the importer, Rerun now:

• loads referenced .mtl files relative to the OBJ file,
• converts each OBJ model into native Mesh3D data,
• preserves positions, triangle indices, normals, and UV coordinates,
• maps MTL Kd/d values to Mesh3D::albedo_factor,
• decodes map_Kd diffuse textures and logs them as Mesh3D albedo textures.

This leaves non-OBJ mesh imports on the existing Asset3D path.

Screenshot

Imported OBJ with its referenced MTL and PNG texture preserved:

Notes

#3772 fixed .obj media-type guessing. The remaining issue in #3773 was that an OBJ file loaded by itself has no access to its MTL/texture sidecars at render time. Resolving those sidecars in the native file importer lets the viewer receive complete Mesh3D archetypes with material data already attached.

Validation

• cargo fmt -p re_importer
• git diff --check
• cargo check -p re_importer
• cargo test -p re_importer --test test_obj_importer
• cargo test -p re_importer
• cargo clippy -p re_importer --all-targets -- -D warnings
• cargo check -p re_view_spatial
• RUST_LOG=error cargo run --package rerun-cli --no-default-features --features release_no_web_viewer -- /Users/Ajab/Downloads/Meshy_AI_Color_Ripple_0710165213_texture_obj/Meshy_AI_Color_Ripple_0710165213_texture.obj --headless --screenshot-to .context/obj-mtl-screenshot/color_ripple_obj_import_wide.png --window-size 1900x1100 --renderer=metal