feat: add OBJ input support

[alexchuber]

Summary

• add ObjInputBlock for OBJ assets, following the existing glTF input contract
• resolve HTTP(S) MTL and relative texture dependencies with redirect support, strict failures, cancellation, and bounded concurrency
• preserve Unicode material names and embed validated texture data for downstream GLB export
• share HTTP data-URI helpers with GltfInputBlock
• document the public behavior and add generated integration fixtures and coverage

Validation

• pnpm test - 67 tests passed
• pnpm lint
• pnpm build
• pnpm format