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/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)