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/PrintObject.cpp

@@ -3391,7 +3391,9 @@ void PrintObject::update_slicing_parameters()
 {
     // Orca: updated function call for XYZ shrinkage compensation
     if (!m_slicing_params.valid) {
-          m_slicing_params = SlicingParameters::create_from_config(this->print()->config(), m_config, this->model_object()->max_z(),
+          coordf_t object_height = this->model_object()->max_z();
+          // Belt printer: height adjustment placeholder (to be implemented in next cycle).
+          m_slicing_params = SlicingParameters::create_from_config(this->print()->config(), m_config, object_height,
                                                                    this->object_extruders(), this->print()->shrinkage_compensation());
       }
 }
@@ -3432,6 +3434,7 @@ SlicingParameters PrintObject::slicing_parameters(const DynamicPrintConfig &full
 
     if (object_max_z <= 0.f)
         object_max_z = (float)model_object.raw_bounding_box().size().z();
+    // Belt printer: height adjustment placeholder (to be implemented in next cycle).
     return SlicingParameters::create_from_config(print_config, object_config, object_max_z, object_extruders, object_shrinkage_compensation);
 }