Desktop release builds: Linux AppImage, Windows exe and macOS app via tools/build_desktop.py

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

INSTALL.md

@@ -53,6 +53,17 @@ data folder: `~/.local/share/dungeoncraft/` (Linux, or `$XDG_DATA_HOME/dungeoncr
 *Save Game* and saved campaigns land in its `saves/` folder; *Load Game* opens the system file dialog there (on
 Linux through the desktop portal), and dropping a file onto the window works as well.
 
+Release packages are built by `python3 tools/build_desktop.py [linux] [windows] [macos]` into `dist/`:
+
+- **Linux**: `DungeonCraft-<version>-x86_64.AppImage`, built against glibc 2.28 (distributions from about 2019 on).
+- **Windows**: `DungeonCraft-<version>-windows-x64.exe`, a single file for Windows 10 and newer, cross-compiled on Linux.
+- **macOS**: `DungeonCraft-<version>-macos.zip` with a universal `DungeonCraft.app` (Apple silicon and Intel); only on a
+  Mac. It is signed ad hoc, not notarised: on first start open it with right click -> *Open*.
+
+Linux and Windows need [zig](https://ziglang.org/) (`mise install zig@0.15` works) and
+`cargo install --locked cargo-zigbuild`, plus `rustup target add x86_64-pc-windows-gnu`; the AppImage tool is downloaded
+once into `~/.cache/dungeoncraft/`.
+
 ### Testing the game logic
 
 The tests run natively: a headless simulation of whole games, a savegame round trip, rendering of all music modules and the audio engine: