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/gig-line.desktop

@@ -0,0 +1,9 @@
+[Desktop Entry]
+Type=Application
+Name=Gig Line
+Comment=Workplace security simulation on Hollywood Boulevard
+Exec=gig-line
+Icon=gig-line
+Terminal=false
+Categories=Game;Simulation;
+Keywords=security;simulation;hollywood;