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/architecture.md

@@ -132,7 +132,7 @@ A continuous-integration pipeline keeps the headless test suite green on every c
 Releases are produced from a tagged commit and published to the repository host's release page, where the README "Downloads" links point.
 
 - `export_presets.cfg` — committed Linux, Windows Desktop, and macOS export presets so builds are reproducible.
-- `tools/build_releases.sh` — exports all three desktop artifacts into `build/`, named `step-line-<platform>-<version>.<ext>`.
+- `tools/build_releases.sh` — exports all three desktop artifacts into `build/`, named `gig-line-<platform>-<version>.<ext>`.
 - `tools/publish_release.sh` — creates a release and uploads the artifacts via the Gitea/Forgejo API (requires `GITEA_TOKEN`).
 - [`RELEASING.md`](RELEASING.md) — step-by-step guide and release checklist.