Add FBX pipeline rules and authoring checklist documentation

[SashaRX]

Summary

• Added comprehensive FBX_PIPELINE_CHECKLIST.md documenting FBX authoring standards, validation rules, and tooling guidance
• Serves as the source of truth for patch_fbx_materials violations and Unity import warnings
• Includes 11 sections covering materials, triangulation, layers, vertex colors, hierarchy, skinning, scene-level rules, naming conventions, tooling compatibility, patcher report categories, and authoring checklist

Changed Zones

• Docs (README.md, CHANGELOG.md)

Checklist

• .meta files present for all new files/directories (documentation file)
• No code changes (documentation only)

Test Plan

N/A — documentation file with no executable code.

Review Notes

This document establishes the contract between FBX authoring tools (Max, Blender, UnityMeshLab) and the patch_fbx_materials patcher. It maps each validation rule to:

• Why the rule exists (Unity import behavior, deformer integrity, etc.)
• Which patch_fbx_materials --report-file categories flag violations
• Which Unity console warnings correspond to each rule
• How to fix violations at source

The document explicitly states that the patcher is a regression net, not the authoring contract — if a tool produces a file that flags any category, the tool is wrong, not the patcher.

Key sections:

• §1–8: Core FBX rules (materials, geometry, layers, naming)
• §9: Tooling guidance (UnityMeshLab, TS_ plugin, MeshLab, Unity FBX Exporter)
• §10: Patcher report category → rule mapping
• §11: Pre-merge checklist for new FBX export code paths

https://claude.ai/code/session_01HCB65PD6V6PrbkuavHGDmQ