Add belt printer transform pipeline: slicing rotation, G-code coords, preview

- Implement core belt slicing pipeline: R(-alpha, X) mesh rotation in PrintObjectSlice with corrected object height calculation for proper layer count
Add to_machine_coords() in GCodeWriter to convert slicing-frame coordinates back to machine-frame, propagated through GCode,
GCodeProcessor, and GCodeViewer
Add belt-mode UI: tilted bed visualization, slicing-direction arrow, and raw G-code toggle to switch between machine-frame and slicing-frame views

This is a combination of 6 commits.

checkpoint 1: initial MVP.  Slicing functions, but rotates instead of skews are happening and a lot of other stuff too

getting somewhere, getting to the point where I need to figure out how to verify this stuff

this appears to be a dead end.

getting somewhere I think maybe

I'm pretty sure we've completely lost the plot at this point and need to restart this process...

remove slice logic in preparation for new, more invasive plan

src/libslic3r/PrintObjectSlice.cpp

@@ -8,6 +8,7 @@
 #include "Layer.hpp"
 #include "MultiMaterialSegmentation.hpp"
 #include "Print.hpp"
+#include "Geometry.hpp"
 //BBS
 #include "ShortestPath.hpp"
 #include "libslic3r/Feature/Interlocking/InterlockingGenerator.hpp"
@@ -139,6 +140,7 @@ static std::vector<VolumeSlices> slice_volumes_inner(
     params_base.closing_radius = print_object_config.slice_closing_radius.value;
     params_base.extra_offset   = 0;
     params_base.trafo          = object_trafo;
+    // Belt printer: mesh transform placeholder (to be implemented in next cycle).
     //BBS: 0.0025mm is safe enough to simplify the data to speed slicing up for high-resolution model.
     //Also has on influence on arc fitting which has default resolution 0.0125mm.
     params_base.resolution = print_config.resolution <= 0.001 ? 0.0f : 0.0025;