Add Blender texcoord mapping value support

[Ahmed-Hindy]

Summary

• add Blender ShaderNodeTexCoord UV-only support by reusing GENERIC::uvmap
• add GENERIC::mapping and GENERIC::value paths for Blender traversal/recreation and USD MaterialX/OpenPBR emission
• emit mapping/value nodes as ND_place2d_vector2 and ND_constant_float, with tests for Blender graph standardization and USD connections

Why

The real Blender scene mo_sh_001_lighting_v005.blend exposed ShaderNodeMapping, ShaderNodeTexCoord, and ShaderNodeValue as unsupported/null nodes in Marble.012. MaterialX has standard equivalents for this UV mapping slice, so those nodes should survive ingestion and USD conversion instead of being dropped.

Validation

• uv --native-tls run pytest tests/test_blender_support.py tests/test_usd_json_conversion.py
• uv --native-tls run pytest
• Headless Blender 4.5 validation on G:\Projects\AYON_PROJECTS\mo_ashraf\Mojar_01\sh_001\work\lighting\mo_sh_001_lighting_v005.blend: 11/11 node materials read and wrote successfully; Marble.012 now reports unsupported_count: 0.

Remaining limitations

• Blender ShaderNodeGroup is still unsupported for felt_fabric_03 and old_gold_04.
• The scene still references two missing external BlenderKit textures under C:\PROJECT\BlenderKit\SCENES\Minimalist podium in front of rocks\Marble\.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 10d736fe-d40b-41cc-a80c-a8163b573086

📥 Commits

Reviewing files that changed from the base of the PR and between a5cdcec and 8b09d44.

📒 Files selected for processing (7)

• src/materials_processor/dcc/blender/recreator.py
• src/materials_processor/dcc/blender/traverser.py
• src/materials_processor/mappings.py
• src/materials_processor/usd/graph_builder.py
• src/materials_processor/usd/mappings.py
• tests/test_blender_support.py
• tests/test_usd_json_conversion.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/blender-texcoord-mapping-value

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