Revive the disabled fff_print test suite (#14196)

* Fix null-deref and arranger bugs that gate headless slicing tests

export_gcode dereferenced a null result out-param, enum serialization
dereferenced a null keys_map, and get_arrange_polys left bed_idx unseeded so
the arranger dropped items. All only affect the headless test/CLI path.

* Fix the headless test harness and add G-code test helpers

Use the real arranger, fix temp-file handling with an RAII guard, and add
layers_with_role / max_z for inspecting sliced G-code.

* Re-enable the Model construction test

* Re-enable SupportMaterial tests and add an enforced-support test

* Re-enable and extend PrintObject layer-height and perimeter tests

* Re-enable Print skirt, brim, and solid-surface tests

* Re-enable and extend PrintGCode tests

Un-hide the basic scenario (dead-key fixes, reframes, trimmed trivia) and add
initial-layer-height, sequential-order, and null-result export tests.

* Re-enable and reframe the skirt/brim tests

Detect skirt/brim by G-code role comment instead of a sentinel speed, and
resolve the previously-unfinished skirt-enclosure test.

* Replace the stale lift()/unlift() test with a z_hop test

* Delete the stub and broken Flow tests

tests/data/fff_print_tests/test_gcodewriter/config_lift_unlift.ini

@@ -1,30 +0,0 @@
-before_layer_gcode = 
-between_objects_gcode = 
-end_filament_gcode = "; Filament-specific end gcode \n;END gcode for filament\n"
-end_gcode = M104 S0 ; turn off temperature\nG28 X0  ; home X axis\nM84     ; disable motors\n
-extrusion_axis = E
-extrusion_multiplier = 1
-filament_cost = 0
-filament_density = 0
-filament_diameter = 3
-filament_max_volumetric_speed = 0
-gcode_comments = 0
-gcode_flavor = reprap
-layer_gcode = 
-max_print_speed = 80
-max_volumetric_speed = 0
-retract_length = 2
-retract_length_toolchange = 10
-retract_lift = 1.5
-retract_lift_above = 0
-retract_lift_below = 0
-retract_restart_extra = 0
-retract_restart_extra_toolchange = 0
-retract_speed = 40
-start_filament_gcode = "; Filament gcode\n"
-start_gcode = G28 ; home all axes\nG1 Z5 F5000 ; lift nozzle\n
-toolchange_gcode = 
-travel_speed = 130
-use_firmware_retraction = 0
-use_relative_e_distances = 0
-use_volumetric_e = 0