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/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