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

.gitea/workflows/build.yml

@@ -77,6 +77,19 @@ jobs:
           chmod +x tools/*.sh
           ./tools/build_releases.sh
 
+      - name: Package Windows installer (NSIS)
+        run: |
+          sudo apt-get update && sudo apt-get install -y nsis
+          VERSION="$(sed -n 's/.*config\/version="\([^"]*\)".*/\1/p' project.godot)"
+          makensis /DVERSION="$VERSION" \
+            /DARTIFACT="build/step-line-windows-$VERSION.exe" \
+            /DICON=packaging/windows/step-line.ico \
+            packaging/windows/step-line.nsi
+
+      - name: Package Linux AppImage
+        run: |
+          bash packaging/linux/build_appimage.sh
+
       - name: Upload installers (workflow_dispatch)
         if: github.event_name == 'workflow_dispatch'
         uses: actions/upload-artifact@v4