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.

packaging/linux/AppRun

@@ -2,4 +2,4 @@
 # AppImage entry point: launch the bundled game binary.
 SELF="$(readlink -f "$0")"
 HERE="$(dirname "$SELF")"
-exec "${HERE}/usr/bin/step-line" "$@"
+exec "${HERE}/usr/bin/gig-line" "$@"