* 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
