Add MeshSave::toPly overloads for TriMesh

[Fedr]

Adds two new functions for saving TriMesh in .ply format:

MRMESH_API Expected<void> toPly( const TriMesh & mesh, const std::filesystem::path& file, const SaveSettings & settings = {} );
MRMESH_API Expected<void> toPly( const TriMesh & mesh, std::ostream & out, const SaveSettings & settings = {} );

To avoid copy-paste and any conversion between Mesh and TriMesh, the existing toPly implementation is turned into the toPlyImpl function template, and the differences between the two representations are factored into small accessor helpers (getVertRenumber, lastValidVert, hasVert, lastValidFace, numValidFaces, hasFace, getTriVerts). All vertices and triangles of TriMesh are considered valid, so vertex renumbering for it is identity, and SaveSettings::onlyValidPoints / SaveSettings::packPrimitives have no effect on it.

The saved bytes for Mesh are exactly the same as before (only mesh.topology.getTriVerts( f, vs ) call was replaced with the equivalent by-value overload).

New unit test MRMesh.TriMeshSavePly verifies that saving a TriMesh produces byte-identical .ply output to saving the equivalent Mesh, and that the output loads back correctly.

Local verification: MRTest Release x64 — all 306 tests pass.

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