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/PrintObject.cpp

@@ -4154,9 +4154,7 @@ void PrintObject::update_slicing_parameters()
           BeltTransformPipeline::BeltFloorParams belt_floor;
           const auto &pcfg = this->print()->config();
           if (pcfg.belt_printer.value) {
-              BoundingBoxf3 bb = BeltTransformPipeline::remap_bbox(*this->model_object(), pcfg);
-              if (BeltTransformPipeline::has_preslice_remap(pcfg))
-                  object_height = bb.size().z();
+              const BoundingBoxf3 bb = this->model_object()->raw_bounding_box();
               auto hr = BeltTransformPipeline::compute_belt_height_and_floor(pcfg, bb, object_height);
               object_height = hr.object_height;
               belt_floor    = hr.floor_params;
@@ -4217,9 +4215,6 @@ SlicingParameters PrintObject::slicing_parameters(const DynamicPrintConfig &full
         BoundingBoxf3 bb = model_object.raw_bounding_box();
         object_max_z = (float)bb.size().z();
         if (print_config.belt_printer.value) {
-            bb = BeltTransformPipeline::remap_bbox(model_object, print_config);
-            if (BeltTransformPipeline::has_preslice_remap(print_config))
-                object_max_z = (float)bb.size().z();
             auto hr = BeltTransformPipeline::compute_belt_height_and_floor(print_config, bb, object_max_z);
             object_max_z = (float)hr.object_height;
             belt_floor   = hr.floor_params;