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")