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/npc_pipeline.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python3
-"""Build the low-poly, skinned NPC base mesh used by Step Line.
+"""Build the low-poly, skinned NPC base mesh used by Gig Line.
 
 The mesh, armature, UVs and neutral material maps are authored procedurally in
 Blender and exported to `assets/models/generated/npc_base.glb`. Godot imports the