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.

docs/RELEASING.md

@@ -1,4 +1,4 @@
-# Releasing Step Line
+# Releasing Gig Line
 
 This guide describes how to produce downloadable executables and publish them
 so the README "Downloads" links always point at the latest release.
@@ -18,7 +18,7 @@ time you export, either:
 Verify with:
 
 ```bash
-godot --headless --export-release "Linux" build/step-line-smoke.x86_64
+godot --headless --export-release "Linux" build/gig-line-smoke.x86_64
 ```
 
 ## 2. Bump the version
@@ -81,17 +81,17 @@ godot --headless --path . --quit-after 120                        # boot smoke t
 
 Artifacts are written to `build/` (all exports embed the PCK — single files):
 
-- `build/step-line-linux-<version>.x86_64`
-- `build/step-line-windows-<version>.exe`
-- `build/step-line-macos-<version>.zip`
-- `build/step-line-web-<version>.zip` — browser build (non-threaded Godot web
+- `build/gig-line-linux-<version>.x86_64`
+- `build/gig-line-windows-<version>.exe`
+- `build/gig-line-macos-<version>.zip`
+- `build/gig-line-web-<version>.zip` — browser build (non-threaded Godot web
   template; unzip and serve the folder, no COOP/COEP headers required)
 
 When run in CI (tags/dispatch), the build job also packages:
 
-- `build/step-line-setup-<version>.exe` — Windows installer (NSIS, incl. shortcuts +
+- `build/gig-line-setup-<version>.exe` — Windows installer (NSIS, incl. shortcuts +
   uninstaller; apt `nsis` on the runner)
-- `build/step-line-<version>-x86_64.AppImage` — Linux AppImage (`packaging/linux/build_appimage.sh`,
+- `build/gig-line-<version>-x86_64.AppImage` — Linux AppImage (`packaging/linux/build_appimage.sh`,
   appimagetool via `APPIMAGE_EXTRACT_AND_RUN`)
 
 Locally you can build the AppImage with `bash packaging/linux/build_appimage.sh <version>`;
@@ -145,7 +145,7 @@ RELEASE_JSON=$(curl -s -X POST "$API" \
 
 RELEASE_ID=$(echo "$RELEASE_JSON" | python3 -c "import sys,json;print(json.load(sys.stdin)['id'])")
 
-for f in build/step-line-*"$VERSION"*; do
+for f in build/gig-line-*"$VERSION"*; do
   curl -s -X POST "$API/$RELEASE_ID/assets" \
     -H "Authorization: token $TOKEN" \
     -F "attachment=@$f"