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/model_pipeline.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python3
-"""Build the low-poly prop models used in the Step Line street scene.
+"""Build the low-poly prop models used in the Gig Line street scene.
 
 The scene needs street furniture (bollards, benches, bins, planters, newspaper
 boxes, a hydrant) that reads as solid geometry instead of flat boxes. This tool
@@ -629,7 +629,7 @@ def build_glb(prop: dict, palette: dict = None, atlas: dict = None) -> bytes:
     mr_view = add_view(atlas["metal_rough_png"])
 
     gltf = {
-        "asset": {"version": "2.0", "generator": "step-line model_pipeline"},
+        "asset": {"version": "2.0", "generator": "gig-line model_pipeline"},
         "scene": 0,
         "scenes": [{"name": prop["name"], "nodes": [0]}],
         "nodes": [{"name": prop["name"], "mesh": 0}],
@@ -889,7 +889,7 @@ def list_props(args) -> int:
 
 def build_parser() -> argparse.ArgumentParser:
     parser = argparse.ArgumentParser(
-        description="Build low-poly prop models for Step Line.")
+        description="Build low-poly prop models for Gig Line.")
     commands = parser.add_subparsers(dest="command", required=True)
 
     listing = commands.add_parser("list", help="print the prop catalogue")