[codex] Add PLY meshfile support

[mickaelbegon]

Summary

Adds PLY meshfile support alongside the existing STL/OBJ/VTP/bioMesh readers.

• Makes meshfile extension dispatch case-insensitive.
• Reworks the ASCII PLY reader to parse the header explicitly, read vertex x/y/z properties, ignore extra properties, and read face vertex_indices / vertex_index lists.
• Triangulates polygonal PLY faces into MeshFace triangles.
• Adds a .PLY fixture and a MeshFile.FileIoPly test covering uppercase extension handling and quad-face triangulation.

Validation

• git diff --check
• Attempted cmake --build build-vitpose-conda, but the local build currently fails in libc++ headers before project code completes (__builtin_ctzg / __builtin_clzg missing). The same toolchain issue appears while compiling unrelated files such as Joints.cpp.
• ctest -N in build-vitpose-conda reports Total Tests: 0.

---

This change is