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/GCode/BeltBackTransform.cpp

@@ -13,16 +13,12 @@ bool BeltBackTransform::init_from_config(const PrintConfig &config)
     if (!config.belt_printer.value || !config.gcode_back_transform.value)
         return false;
 
-    // Require at least one active transform to proceed.
-    bool has_global_rotation = config.belt_slice_rotation_global.value
-                            && config.belt_slice_rotation.value != BeltRotationAxis::None;
-    bool has_preslice_global = config.belt_preslice_global.value
-                            || config.preslice_remap_global.value;
-    if (!has_global_rotation && !has_preslice_global
-        && !BeltTransformPipeline::has_preslice_remap(config))
+    // The back-transform undoes the global pre-slice rotation; without global
+    // mode the slicing frame is not a common frame to undo.
+    if (!config.belt_preslice_global.value)
         return false;
 
-    // Build the forward pipeline (rotation * pre_remap) and store its inverse.
+    // Build the forward pipeline (the rotation) and store its inverse.
     Transform3d forward = BeltTransformPipeline::build_forward_transform(config);
     if (forward.isApprox(Transform3d::Identity()))
         return false;