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.

tests/fff_print/test_gcodewriter.cpp

@@ -617,6 +617,9 @@ static DynamicPrintConfig dual_extruder_toolchange_config()
     config.set_key_value("nozzle_temperature_range_high", new ConfigOptionInts({240, 240}));
     config.set_key_value("flush_multiplier",     new ConfigOptionFloats({1}));
     config.set_key_value("flush_volumes_matrix", new ConfigOptionFloats({0, 140, 140, 0}));
+    // Inside the 200x200 test bed; the default y, 220, is not, and generation rejects that.
+    config.set_key_value("wipe_tower_x",         new ConfigOptionFloats({50.}));
+    config.set_key_value("wipe_tower_y",         new ConfigOptionFloats({50.}));
     return config;
 }