Merge upstream/hanif/belt-printer-fixes into belt/final-round

Brings in upstream/belt-printer (the Sept 14 main merge) plus Hanif Koh's
21 review-fix commits from PR #15685, on top of the MachineKinematics
refactor and the purge-prism / tree-support / first-layer-speed fixes.

Conflict resolution:
- BeltGCodeWriter is gone (kinematics refactor), so Hanif's plate-offset
  fix for it is ported into GCodeWriter: the first-layer-plane checks in
  travel_to_xy / travel_to_xyz / _travel_to_z now evaluate the plate-local
  point, and BeltGCode::init_belt_writer hands the stored plate origin to
  the writer it installs.
- init_belt_writer(Print&) takes Hanif's signature; the BBL flag is set on
  the surviving writer by GCode::_do_export.
- The shared emit_belt_brim_bands() loop keeps the BeltFloorObjectGuard the
  local branch added, so apron bands classify first-layer height against
  their own object.
- eager_lift keeps effective_type: it now carries set_force_normal_lift().
- GCodeWriter's initializer list follows Hanif's member order with
  m_kinematics in its declared position.
- TreeSupport::detect_overhangs uses Hanif's clamped build_plate_tilt_slope()
  for the non-belt path and the belt shear for the belt path.

src/libslic3r/calib.cpp

@@ -29,9 +29,9 @@ std::string CalibPressureAdvance::move_to(Vec2d pt, GCodeWriter &writer, std::st
 
     gcode << writer.retract(); // retract before z move or move
     if(z > EPSILON && layer_height >= 0){
-        gcode << writer.travel_to_z(z, "z-hop"); // Perform z hop
+        gcode << writer.travel_to_z(z, "Z-hop"); // Perform z hop
         gcode << writer.travel_to_xy(pt, comment); // Travel with z move
-        gcode << writer.travel_to_z(layer_height, "undo z-hop"); // Undo z hop
+        gcode << writer.travel_to_z(layer_height, "undo Z-hop"); // Undo z hop
     }else {
         gcode << writer.travel_to_xy(pt, comment);
     }