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/PrintApply.cpp

@@ -1844,12 +1844,7 @@ Print::ApplyStatus Print::apply(const Model &model, DynamicPrintConfig new_full_
             // Orca: Updated for XYZ filament shrink compensation
             // Belt global mode: force each instance into its own PrintObject
             // so each gets independent layer Z values.
-            bool belt_force_separate = m_config.belt_printer.value && (
-                (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)
-                || m_config.belt_preslice_global.value
-                || (m_config.preslice_remap_global.value && BeltTransformPipeline::has_preslice_remap(m_config)));
+            bool belt_force_separate = m_config.belt_printer.value && m_config.belt_preslice_global.value;
             model_object_status.print_instances = print_objects_from_model_object(*model_object, this->shrinkage_compensation(), belt_force_separate);
             std::vector<const PrintObjectStatus*> old;
             old.reserve(print_object_status_db.count(*model_object));
@@ -1940,11 +1935,7 @@ Print::ApplyStatus Print::apply(const Model &model, DynamicPrintConfig new_full_
         // min_shift across all objects, so one move affects everyone).
         if (belt_instances_shifted
             && m_config.belt_printer.value
-            && ((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)
-                || m_config.belt_preslice_global.value
-                || (m_config.preslice_remap_global.value && BeltTransformPipeline::has_preslice_remap(m_config)))) {
+            && m_config.belt_preslice_global.value) {
             for (PrintObject *object : m_objects)
                 update_apply_status(object->invalidate_step(posSlice));
         }