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

export_presets.cfg

@@ -23,7 +23,7 @@ script_export_mode=2
 custom_template/debug=""
 custom_template/release=""
 debug/export_console_wrapper=1
-binary_format/embed_pck=false
+binary_format/embed_pck=true
 texture_format/s3tc_bptc=true
 texture_format/etc2_astc=false
 binary_format/architecture="x86_64"
@@ -54,7 +54,7 @@ script_export_mode=2
 custom_template/debug=""
 custom_template/release=""
 debug/export_console_wrapper=0
-binary_format/embed_pck=false
+binary_format/embed_pck=true
 texture_format/s3tc_bptc=true
 texture_format/etc2_astc=false
 codesign/enable=false
@@ -90,7 +90,7 @@ script_export_mode=2
 custom_template/debug=""
 custom_template/release=""
 debug/export_console_wrapper=0
-binary_format/embed_pck=false
+binary_format/embed_pck=true
 texture_format/s3tc_bptc=true
 texture_format/etc2_astc=true
 codesign/enable=false