Mesh IO: Add multi-chart (multi-texture) support to PLY writes

[csparker247]

Background

The mesh-io work (#14) intentionally shipped PLY texture support as single-chart only. write_obj has a std::vector<std::filesystem::path> overload that groups faces by chart and emits per-chart usemtl material0/material1/... (with a matching .mtl), but write_ply has no vector overload — it emits a single comment TextureFile <path> line.

This was a deliberate scope decision at the time, recorded in conductor/tracks/mesh-io_20260323/spec.md (Out of Scope: "Multi-texture PLY write (no well-supported standard outside MeshLab)") on the grounds that multi-texture PLY has no broadly-supported ecosystem standard.

Note the asymmetry already in place: read_ply already recovers multiple comment TextureFile lines into its texture_paths out-parameter (MeshLab convention, covered by PLYTest.ReadCommentTextureFile_HandCraftedPLY). So PLY can already read multi-chart texture references — it just can't write them.

Proposal

Add a multi-chart write_ply overload mirroring the OBJ one:

void write_ply(
    const std::filesystem::path& path,
    const Mesh<T, Dims, VTraits>& mesh,
    const UVMapT& uvmap,
    const std::vector<std::filesystem::path>& texture_paths);

• static_assert(traits::has_chart<UVMapT>::value, ...), matching the multi-chart write_obj contract.
• Emit one comment TextureFile <path> line per texture, in chart-index order, immediately after the format line (the same layout read_ply already parses).
• Per-wedge UVs continue to go in the face texcoord property as today; charts are distinguished by the chart index on each UV coordinate. Decide how/whether to record per-face chart assignment in the PLY body (e.g. a per-face chart/material_index scalar property) so a written multi-chart PLY round-trips chart indices, not just texture paths.

Acceptance criteria

• write_ply vector overload exists and static_asserts on traits::has_chart.
• Writes one comment TextureFile line per chart, in index order.
• Round-trip test: write a 2-chart mesh, read it back, recover both texture paths and per-corner chart indices.
• Single-chart behavior and the existing single-path overload are unchanged.
• Doxygen @notes on write_ply/read_ply that currently say "PLY write is single-texture only" are updated.
• read_mesh/write_mesh facade dispatches the new overload for .ply.

References

• Follow-up to Mesh IO: OBJ and PLY read/write #14
• include/educelab/core/io/MeshIO_PLY.hpp — write_ply Tier 3 (single-path)
• include/educelab/core/io/MeshIO_OBJ.hpp:724 — write_obj multi-chart overload (reference implementation)
• tests/src/TestMeshIO.cpp — OBJTest.PositionsWithUVs_MultiChart_PerChartUsemtl (reference test)