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

DONE.md

@@ -185,6 +185,22 @@ Completed task sections moved from `TODO.md`. Sections are moved only when every
       Follow-up: anonymous **asset downloads** require sign-in on this host (host setting).
 - [x] CI packaging follow-up was already live (`.gitea/workflows/build.yml` on tags).
 
+## 📦 Installer packaging (Windows NSIS + Linux AppImage)
+
+- [x] Export presets: `binary_format/embed_pck=true` on all platforms — every binary is now
+      self-contained (all game data in one file; `.pck` no longer shipped).
+- [x] App icon: `assets/textures/icon.png` (1024) + multi-size `icon.ico`, copied to
+      `packaging/windows/step-line.ico` and `packaging/linux/step-line.png` via the generator.
+- [x] NSIS script `packaging/windows/step-line.nsi` (installs `Step Line.exe`, uninstaller,
+      shortcuts, registry entry; `makensis /DVERSION=… /DARTIFACT=…`).
+- [x] AppImage: `packaging/linux/step-line.desktop`, `AppRun`, appdata.xml, and
+      `build_appimage.sh` (appimagetool, `APPIMAGE_EXTRACT_AND_RUN=1`).
+- [x] Local end-to-end: rebuilt embedded exports and produced `step-line-0.2.0-x86_64.AppImage`,
+      ran it headless (0 script errors).
+- [x] CI: NSIS + AppImage steps added to `.gitea/workflows/build.yml` build job; publish glob
+      already uploads `build/step-line-*<ver>*` (covers setup/AppImage names).
+- [ ] (open) CI run verification of the NSIS build and installer-content listing.
+
 ## 🎮 Assets & detailed graphics pass (textures, rigs, UI feedback)
 
 - [x] Authored texture set via `tools/generate_textures.py` (deterministic PIL): brick facade,