[io] Support large polygons in PLY output

[fallenmi]

Fixes #6087.

Summary

• keep uchar face-list counts for meshes whose polygons have at most 255 vertices
• switch the whole face-list property to uint when any polygon is larger
• write matching 32-bit counts for every binary face in promoted files
• add ASCII and binary round-trip coverage at the 255/256 boundary

Why

PLY declares one list-count type per face property. A 256-vertex polygon was previously declared as uchar; in binary output its size was cast to zero, corrupting the following index stream. The mesh-wide choice preserves compatibility for ordinary files while making mixed small/large meshes self-consistent.

Validation

• regression against master: failed on the missing uint header and ASCII parse mismatch
• patched focused regression: 10 repeated passes
• full test_ply_io: 74/74 passed
• git diff --check
• local macOS arm64 Debug build with VTK, OpenNI, CUDA, and Qhull disabled

AI assistance disclosure: OpenAI Codex helped investigate and draft this change; I reproduced the bug, reviewed the patch, and ran the tests listed above.