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/generate_textures.py

@@ -159,8 +159,7 @@ def vignette():
             px[x, y] = int(255 * (t * t))
     save(img, "vignette.png")
 
-def icon(name, draw_fn):
-    size = 64
+def icon(name, draw_fn, size=64):
     img = Image.new("RGBA", (size, size), (0, 0, 0, 0))
     d = ImageDraw.Draw(img)
     draw_fn(d, size)
@@ -185,6 +184,30 @@ def icons():
     icon("icons/wand.png", wand)
     icon("icons/rec_dot.png", rec)
 
+def app_icon():
+    """Game app icon: dark badge, gold star, white 'S' — used by desktop/AppImage
+    (PNG) and the Windows installer (ICO)."""
+    def badge(d, s):
+        d.ellipse([2, 2, s-2, s-2], fill=(24, 26, 38, 255))
+        d.ellipse([int(s*0.10), int(s*0.10), int(s*0.90), int(s*0.90)], outline=(120, 130, 170, 255), width=max(2, s//64))
+        # gold star
+        cx, cy, r = s*0.5, s*0.44, s*0.30
+        pts = []
+        for i in range(10):
+            ang = -90 + i * 36
+            rad = r if i % 2 == 0 else r * 0.42
+            import math
+            pts.append((cx + rad * math.cos(math.radians(ang)), cy + rad * math.sin(math.radians(ang))))
+        d.polygon(pts, fill=(255, 205, 80, 255))
+    icon("icon.png", badge, size=1024)
+    icon("icon.ico", badge, size=256)
+
+    # multi-size ICO for Windows (from the 256 render)
+    src = Image.open(os.path.join(ASSETS, "icon.png"))
+    ico_path = os.path.join(ASSETS, "icon.ico")
+    src.save(ico_path, format="ICO", sizes=[(256, 256), (128, 128), (64, 64), (48, 48), (32, 32), (16, 16)])
+    print("wrote assets/textures/icon.ico (multi-size)")
+
 def cards():
     CW, CH = 256, 128
     def base(card_name, bg, label, band=(0,0,0,0)):
@@ -220,5 +243,6 @@ if __name__ == "__main__":
     skyline()
     vignette()
     icons()
+    app_icon()
     cards()
     print("done")