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

README.md

@@ -20,16 +20,19 @@ The central design goal is simple:
 
 | Platform | Package |
 | --- | --- |
-| **Windows** | [`step-line-windows.exe`](https://git.allucanget.biz/zwitschi/security-game/releases) · 64-bit, no install needed |
-| **Linux** | [`step-line-linux.x86_64`](https://git.allucanget.biz/zwitschi/security-game/releases) · 64-bit, `chmod +x` and run |
-| **macOS** | [`step-line-macos.zip`](https://git.allucanget.biz/zwitschi/security-game/releases) · unsigned build, quarantine note below |
+| **Windows — installer** | [`step-line-setup-<ver>.exe`](https://git.allucanget.biz/zwitschi/security-game/releases) · contains all game data, shortcuts + uninstaller |
+| **Windows — portable** | [`step-line-windows-<ver>.exe`](https://git.allucanget.biz/zwitschi/security-game/releases) · 64-bit, no install needed |
+| **Linux — AppImage** | [`step-line-<ver>-x86_64.AppImage`](https://git.allucanget.biz/zwitschi/security-game/releases) · single file, `chmod +x` and run |
+| **Linux — portable** | [`step-line-linux-<ver>.x86_64`](https://git.allucanget.biz/zwitschi/security-game/releases) · 64-bit, `chmod +x` and run |
+| **macOS** | [`step-line-macos-<ver>.zip`](https://git.allucanget.biz/zwitschi/security-game/releases) · unsigned build, quarantine note below |
 
 **All releases:** <https://git.allucanget.biz/zwitschi/security-game/releases>
 
 > **Latest: v0.2.0** (2026-10-31) — built and published by Gitea Actions from the
 > `v0.2.0` tag. Releases are built automatically when a `vX.Y.Z` tag is pushed
 > (see [`docs/RELEASING.md`](docs/RELEASING.md)); manual builds use
-> `./tools/build_releases.sh`.
+> `./tools/build_releases.sh`. Installer/AppImage packaging runs in the same pipeline
+> (NSIS + appimagetool) and uploads with the portable binaries.
 >
 > Note: on this host, downloading release files currently requires sign-in;
 > the page and tag URLs are public.