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/slic3r/GUI/GCodeViewer.cpp

@@ -1339,10 +1339,9 @@ static Transform3d compute_belt_back_transform(const PrintConfig& cfg)
     // build-volume offset for Rev axes). This is the matrix form of the per-point
     // GCodeWriter::apply_axis_remap (row convention: each OUTPUT axis selects an input
     // axis + sign) and MUST stay in sync with it. The build-volume max matches what the
-    // writer is fed in GCode.cpp (printable_area max + printable_height). NB: this is the
-    // transpose of the column convention used by BeltTransformPipeline::build_preslice_remap
-    // — the two remaps are not interchangeable. (Follow-up: precompute this matrix once in
-    // GCodeWriter and share it with apply_axis_remap to remove the parallel encoding.)
+    // writer is fed in GCode.cpp (printable_area max + printable_height). (Follow-up:
+    // precompute this matrix once in GCodeWriter and share it with apply_axis_remap to
+    // remove the parallel encoding.)
     Transform3d ar = Transform3d::Identity();
     const int rr[3] = { int(cfg.gcode_remap_x.value), int(cfg.gcode_remap_y.value), int(cfg.gcode_remap_z.value) };
     if (rr[0] != 0 || rr[1] != 1 || rr[2] != 2) {