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

tools/build_releases.sh

@@ -16,6 +16,8 @@ set -euo pipefail
 GODOT="${GODOT:-godot}"
 VERSION="$(grep -oP 'config/version="\K[^"]+' project.godot)"
 mkdir -p build
+# Presets now embed the PCK into a single binary; drop stale .pck leftovers.
+rm -f build/*.pck
 
 echo "==> Exporting Linux (x86_64)"
 "$GODOT" --headless --export-release "Linux" "build/step-line-linux-${VERSION}.x86_64"