Add Windows cross-build and Gitea release; share CI setup

- just windows-deps / windows / package-windows: cross-compile with Odin
  objects + lld-link against an xwin SDK, static VC runtime
- Release workflow runs on v* tags, builds Linux and Windows packages,
  uploads them to the package registry and creates a Gitea release
- Shared toolchain setup as a local composite action

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

AGENTS.md

@@ -33,6 +33,7 @@ Commands (see `justfile`): `just run`, `just check`, `just test`, `just package
 2. **Old font failure root cause**: `DroidSansJapanese.ttf` was a GitHub HTML page saved from a blob URL, not a font. Always download fonts from raw URLs and check them with `file`.
 3. **Thai shaping**: raylib does no text shaping, so Thai combining vowels/tone marks may be misplaced. Prefer standalone consonants on Thai cards.
 4. Tests share the memory game's global state, so they must run single-threaded (`just test` sets `ODIN_TEST_THREADS=1`).
+6. **Windows builds are cross-compiled**: Odin can't link Windows binaries on Linux, so `just windows` builds objects with `-build-mode:obj` and links with `lld-link` against an xwin SDK. The VC runtime is linked statically; only Windows system DLLs are needed. Not yet tested on a real Windows machine.
 5. The game changes directory to the executable's folder at startup so `assets/` is found from anywhere.
 
 ## File Structure
@@ -43,9 +44,11 @@ HanjaKanaMun/
 ├── justfile               # build/run/check/test/package recipes
 ├── ols.json               # ols language server settings (neovim)
 ├── odinfmt.json           # odinfmt formatter settings (4 spaces, LF)
-├── .gitea/workflows/
-│   ├── gitea-build.yaml    # check + test + build on main/develop
-│   └── gitea-release.yaml  # package on version branches (*.*), upload to Gitea packages
+├── .gitea/
+│   ├── actions/setup/      # shared CI toolchain setup (apt, Odin, just)
+│   └── workflows/
+│       ├── gitea-build.yaml    # check + test + build on main/develop
+│       └── gitea-release.yaml  # on tags v*: Linux + Windows packages, package registry upload, Gitea release
 ├── main.odin              # Window/audio setup, screen state machine
 ├── src/
 │   ├── themes/themes.odin  # Theme struct, flag colors, Screen/Flag enums, layout constants