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

src/libslic3r/ModelArrange.cpp

@@ -19,6 +19,10 @@ arrangement::ArrangePolygons get_arrange_polys(const Model &model, ModelInstance
     for (ModelObject *mo : model.objects)
         for (ModelInstance *minst : mo->instances) {
             minst->get_arrange_polygon(&ap);
+            // ModelInstance::get_arrange_polygon leaves bed_idx at its UNARRANGED
+            // default; seed it to bed 0 (as get_instance_arrange_poly does) so the
+            // nester treats the item as placeable instead of returning it unplaced.
+            ap.bed_idx = 0;
             input.emplace_back(ap);
             instances.emplace_back(minst);
         }