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.

project.godot

@@ -10,7 +10,7 @@ config_version=5
 
 [application]
 
-config/name="Step Line"
+config/name="Gig Line"
 config/description="Workplace security simulation about observation, access verification, and proportionate escalation."
 config/version="0.5.0"
 run/main_scene="res://scenes/main.tscn"