[T3] No round-trip IO test for OBJ and PLY formats

[csparker247]

Summary

Reading a mesh, writing it, and reading it back should produce an identical mesh. No such round-trip test exists for OBJ or PLY.

Required Tests

Using in-memory std::ostringstream/std::istringstream (no temp files):

• TEST(MeshIO, OBJ_RoundTrip): write pyramid → read back → assert identical vertex positions and face connectivity
• TEST(MeshIO, PLY_RoundTrip): same for PLY

Conductor Track

T3