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/slic3r/GUI/GCodeViewer.hpp

@@ -237,6 +237,9 @@ private:
 mutable bool m_no_render_path { false };
     bool m_is_dark = false;
 
+    bool  m_belt_view_enabled = false;
+    float m_belt_angle_deg = 0.f;
+
     libvgcode::Viewer m_viewer;
     bool m_loaded_as_preview{ false };
 
@@ -336,6 +339,9 @@ public:
 
     void export_toolpaths_to_obj(const char* filename) const;
 
+    void set_belt_printer(bool enabled, float angle_deg) { m_belt_view_enabled = enabled; m_belt_angle_deg = angle_deg; }
+    bool is_belt_view() const { return m_belt_view_enabled && m_belt_angle_deg > 0.f; }
+
     size_t get_extruders_count() { return m_extruders_count; }
     void push_combo_style();
     void pop_combo_style();