Variable dropUnusedMaterialConst

dropUnusedMaterial: Fix = ...

Auto-fix for MATERIAL_UNUSED — destroys a SceneMaterial that no SceneMesh references. The fix checks live mesh references by object identity before destroying; this lets it clean up models whose numMeshes counter was left stale by a replaced same-id material pointer.

Idempotent: returns {fixed: false} when the material is already destroyed or absent.