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.

tools/screen_designs.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python3
-"""Generate screen-design reference images for Step Line and compare captures.
+"""Generate screen-design reference images for Gig Line and compare captures.
 
 This tool turns the screen descriptions that already live in `docs/`
 (README screenshot captions, ADRs 0007-0010, PLAN.md, architecture.md, and the
@@ -507,7 +507,7 @@ SCREENS = [
         "prompt": (
             "First-person 3D game screenshot at night from a security guard standing at "
             "the bottom of venue steps, looking at a lit entrance door with a striped "
-            "awning and STEP LINE sign. Subtle head-height view, brick facade, dark blue "
+            "awning and GIG LINE sign. Subtle head-height view, brick facade, dark blue "
             "night sky, minimal HUD: a small cyan 'Precision walk (Shift)' chip at the "
             "bottom left, shift clock top right, thin control hint line at the bottom. "
             "Realistic lighting, muted colours, no people"),
@@ -944,7 +944,7 @@ def _call_openrouter(body, timeout=180):
             "Authorization": f"Bearer {key}",
             "Content-Type": "application/json",
             "HTTP-Referer": "https://git.allucanget.biz/zwitschi/security-game",
-            "X-Title": "Step Line",
+            "X-Title": "Gig Line",
         },
         method="POST",
     )
@@ -1018,7 +1018,7 @@ def judge(args):
 
 def build_parser():
     parser = argparse.ArgumentParser(
-        description="Generate and compare Step Line screen-design references.")
+        description="Generate and compare Gig Line screen-design references.")
     commands = parser.add_subparsers(dest="command", required=True)
 
     gen = commands.add_parser(