Add Windows installer (NSIS) and Linux AppImage packaging; embed PCK in all exports

- export_presets: embed_pck=true for Linux/Windows/macOS -> single self-contained
  binaries (all game data in one file); build_releases.sh cleans stale .pck
- App icon: assets/textures/icon.png (1024) + multi-size icon.ico generated by
  tools/generate_textures.py; copied into packaging/windows and packaging/linux
- packaging/windows/step-line.nsi: NSIS installer (shortcuts, uninstaller,
  registry Add/Remove Programs) output step-line-setup-<ver>.exe
- packaging/linux: step-line.desktop, AppRun, step-line.appdata.xml,
  build_appimage.sh (appimagetool, APPIMAGE_EXTRACT_AND_RUN) ->
  step-line-<ver>-x86_64.AppImage
- CI build job now packages NSIS installer + AppImage alongside binaries;
  publish glob already uploads them
- Local end-to-end verification: embedded exports rebuilt, AppImage built and
  run headless (0 script errors); suite 57/57
- README Downloads table lists installer/AppImage/portable rows; RELEASING
  documents packaging; TODO/DONE synced

docs/RELEASING.md

@@ -57,12 +57,22 @@ godot --headless --path . --quit-after 120                        # boot smoke t
 ./tools/build_releases.sh
 ```
 
-Artifacts are written to `build/`:
+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`
 
+When run in CI (tags/dispatch), the build job also packages:
+
+- `build/step-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`,
+  appimagetool via `APPIMAGE_EXTRACT_AND_RUN`)
+
+Locally you can build the AppImage with `bash packaging/linux/build_appimage.sh <version>`;
+the NSIS installer requires `makensis` (covered by CI).
+
 > macOS builds are unsigned and will need `xattr -dr com.apple.quarantine`
 > guidance or notarization for production releases.