Simpe-Windows-0.8.1.6

GMDC preview: vertex colors, joint overlay, mesh transforms + port notes

- Fix matrix multiply order in MatrixStack.MultiplyMatrixLocal (new * top,
  not top * new) so stacked transforms compose correctly
- Respect per-vertex color: GlMesh now tracks HasVertexColors; DirectXPanel
  enables vertex-color path only when the mesh actually carries colors
- SceneToMesh copies Translation/Rotation/Scaling from source to dest mesh
  so GMDCs with non-identity mesh transforms render in the right place
- Joint meshes render with ZTest=false so the skeleton is visible through
  the body; bone thickness scaled to joint size (linewd: num * 0.2)
- RenderSelection flips RenderJoints on/off based on listbox selection
- Compute projection aspect from vp.Aspect (with >0 guard) instead of
  re-reading control dimensions; set GL viewport + aspect at init time
- Default camoffset 0.7 -> 1.0, jsz 1 -> 15, AddAxis true, RenderJoints
  false by default (user toggles by selecting a joint)
- Add DumpPreviewDiagnostic writing panel/camera/mesh state to desktop
  for debugging
- Add GMDC_PREVIEW_PORT_NOTES.md — full handoff doc for the Mac/Avalonia