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/main.gd

@@ -8,4 +8,4 @@ func _ready() -> void:
 	var manager := GameManager.new()
 	manager.name = "GameManager"
 	add_child(manager)
-	print("[Step Line] game started")
+	print("[Gig Line] game started")

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%%" % [

src/game/ui/start_menu.gd

@@ -47,7 +47,7 @@ func _build() -> void:
 	center.add_child(column)
 
 	var title := Label.new()
-	title.text = "STEP LINE"
+	title.text = "GIG LINE"
 	title.add_theme_font_size_override("font_size", 44)
 	title.horizontal_alignment = HORIZONTAL_ALIGNMENT_CENTER
 	column.add_child(title)

src/game/world_builder.gd

@@ -270,7 +270,7 @@ func _build_marquee() -> void:
 		_marquee_emissive.append(mat)
 	marquee_label = Label3D.new()
 	marquee_label.name = "MarqueeLabel"
-	marquee_label.text = "STEP LINE"
+	marquee_label.text = "GIG LINE"
 	marquee_label.position = Vector3(0, 7.3, -3.55)
 	marquee_label.modulate = Color(1, 0.6, 0.9)
 	marquee_label.font_size = 160