Belt printer: retire the redundant and unused options

Removed, with the keys added to handle_legacy()'s ignore list so saved
profiles and 3MFs keep loading:

- belt_slice_rotation_global and preslice_remap_global. Both were only
  consulted when belt_preslice_global ("Global mesh transforms") was off,
  which no profile does; belt_preslice_global is now the single global
  mode and is presumed on everywhere the old flags were ORed in
  (PrintObjectSlice, BeltBackTransform, BeltGCode, Print::process,
  PrintApply). The Belt tilt row is axis + angle only.
- preslice_remap_x/y/z. No profile used the pre-slice axis remap; the belt
  tilt axis plus the G-code axis remap cover the machines that exist, and
  its implementation only agreed with itself for a plain swap (matrix
  columns vs remap_bbox rows). BeltTransformPipeline::build_preslice_remap,
  remap_bbox and has_preslice_remap are gone, the forward transform is the
  rotation, and the G-code header no longer carries the remap.
- belt_support_z_offset_mode. Saved and invalidated steps, but no support
  generator read it.
- first_layer_plane and first_layer_plane_offset, with FirstLayerPlane.cpp.
  On every shipped configuration the band is measured from the belt
  surface (GCode::belt_height_above_floor) and the evaluator was only
  reached for an explicit XY/YZ/XZ choice or a non-zero offset, which
  nobody set. first_layer_plane_thickness stays as the band unit,
  relabelled "First layer band thickness".

UI: the Machine frame transforms group is five single-option rows (G-code
remap X / Y / Z, Decouple machine-frame tilt, Machine-frame tilt angle;
the angle row is shown only when decoupled) instead of two multi-column
lines, and the remap fields carry full labels.

Also carries the phong.fs struct fix from #16226 so the worktree build
links its shaders.

libslic3r_tests and fff_print_tests pass; clang-tidy diff check clean;
orca_profile_tool.py check clean.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/libslic3r/BeltGCode.cpp

@@ -25,12 +25,6 @@ void BeltGCode::write_belt_header(GCodeOutputStream &file, const Print &print)
     // for the physical tilt the G-code viewer uses to enable belt view.
     file.write_format("; belt_slice_rotation = %s\n", full_cfg.opt_serialize("belt_slice_rotation").c_str());
     file.write_format("; belt_slice_rotation_angle = %.1f\n", print.config().belt_slice_rotation_angle.value);
-    file.write_format("; belt_slice_rotation_global = %d\n", print.config().belt_slice_rotation_global.value ? 1 : 0);
-    // Pre-slice remap configs
-    file.write_format("; preslice_remap_x = %s\n", full_cfg.opt_serialize("preslice_remap_x").c_str());
-    file.write_format("; preslice_remap_y = %s\n", full_cfg.opt_serialize("preslice_remap_y").c_str());
-    file.write_format("; preslice_remap_z = %s\n", full_cfg.opt_serialize("preslice_remap_z").c_str());
-    file.write_format("; preslice_remap_global = %d\n", print.config().preslice_remap_global.value ? 1 : 0);
     file.write_format("; belt_preslice_global = %d\n", print.config().belt_preslice_global.value ? 1 : 0);
     // Machine-frame transform: shear (tan) + scale (1/cos) derived from the belt
     // tilt angle (or belt_frame_tilt_angle when decoupled).
@@ -40,25 +34,12 @@ void BeltGCode::write_belt_header(GCodeOutputStream &file, const Print &print)
 
 void BeltGCode::on_set_origin(const PrintObject * /*obj*/, const Point & /*inst_shift*/)
 {
-    // Global pre-slice mode: adjust origin using computed correction.
-    // Transform the origin through the belt pipeline so that
-    // back_transform(T * origin) = origin (correct machine position).
-    //
-    // Flags that trigger this path:
-    //   belt_preslice_global         — full pipeline (rotation * remap) is global
-    //   preslice_remap_global        — only the pre-slice remap is global
-    //   belt_slice_rotation_global   — slicing rotation treated as global (matches
-    //                                  the per-instance Z-offset added in PrintObjectSlice.cpp)
-    // The XY origin adjustment uses the FULL forward transform, because the
-    // back_transform applied during G-code emission is always the inverse of
-    // the full pipeline.
-    bool use_global = m_config.belt_preslice_global.value
-        || (m_config.preslice_remap_global.value
-            && BeltTransformPipeline::has_preslice_remap(m_config))
-        || (m_config.belt_slice_rotation_global.value
-            && m_config.belt_slice_rotation.value != BeltRotationAxis::None
-            && std::abs(m_config.belt_slice_rotation_angle.value) > EPSILON);
-    if (!use_global)
+    // Global pre-slice mode (matches the per-instance Z-offset added in
+    // PrintObjectSlice.cpp): transform the origin through the belt pipeline so
+    // that back_transform(T * origin) = origin (correct machine position). The
+    // back_transform applied during G-code emission is the inverse of the
+    // forward transform.
+    if (!m_config.belt_preslice_global.value)
         return;
 
     // Adjust origin: transform through belt forward pipeline so that