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/Print.cpp

@@ -565,6 +565,9 @@ std::vector<ObjectID> Print::print_object_ids() const
 
 bool Print::has_infinite_skirt() const
 {
+    // Belt printer: no skirt support.
+    if (m_config.belt_printer.value)
+        return false;
     // Orca: unclear why (m_config.ooze_prevention && this->extruders().size() > 1) logic is here, removed.
     // return (m_config.draft_shield == dsEnabled && m_config.skirt_loops > 0) || (m_config.ooze_prevention && this->extruders().size() > 1);
 
@@ -573,6 +576,9 @@ bool Print::has_infinite_skirt() const
 
 bool Print::has_skirt() const
 {
+    // Belt printer: no skirt support.
+    if (m_config.belt_printer.value)
+        return false;
     return (m_config.skirt_height > 0);
 }
 
@@ -1194,6 +1200,16 @@ StringObjectException Print::validate(StringObjectException *warning, Polygons*
     if (extruders.empty())
         return { L("No extrusions under current settings.") };
 
+    // Belt printer validation: incompatible features.
+    if (m_config.belt_printer.value) {
+        for (const PrintObject *object : m_objects) {
+            if (object->config().raft_layers > 0)
+                return { L("Raft is not compatible with belt printer mode.") };
+        }
+        if (m_config.draft_shield != dsDisabled)
+            return { L("Draft shield is not compatible with belt printer mode.") };
+    }
+
     if (nozzles < 2 && extruders.size() > 1) {
         auto ret = check_multi_filament_valid(*this);
         if (!ret.string.empty())
@@ -2501,6 +2517,10 @@ std::string Print::export_gcode(const std::string& path_template, GCodeProcessor
 
 void Print::_make_skirt()
 {
+    // Belt printer: skirt is not compatible.
+    if (m_config.belt_printer.value)
+        return;
+
     // First off we need to decide how tall the skirt must be.
     // The skirt_height option from config is expressed in layers, but our
     // object might have different layer heights, so we need to find the print_z