Rename Step Line to Gig Line

Update the game name in project.godot, export presets, UI/marquee/title strings, packaging (AppImage/NSIS/desktop/appdata), tools, docs, and the generated prop GLBs. Packaging assets and README screenshots are regenerated. Headless suite 209/209 and Python tooling tests pass.

src/game/ui/hud.gd

@@ -266,7 +266,7 @@ func _init() -> void:
 	add_child(debug_label)
 
 func set_shift_state(coordinator: Object, scenario: Resource) -> void:
-	top_label.text = scenario.display_name if scenario else "Step Line"
+	top_label.text = scenario.display_name if scenario else "Gig Line"
 	var mins := int(coordinator.elapsed_s) / 60
 	var secs := int(coordinator.elapsed_s) % 60
 	time_label.text = "shift %02d:%02d  phase %s\nrep %.0f  crowd %.0f%%" % [