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>

.gitea/actions/setup/action.yaml

@@ -0,0 +1,44 @@
+#
+# .gitea/actions/setup/action.yaml
+#
+# Installs the toolchain shared by all workflows: system packages, Odin and just.
+#
+
+name: "Setup Odin toolchain"
+description: "Install system packages, Odin and just"
+
+inputs:
+  odin-version:
+    description: "Odin release tag (keep in sync with odin_version in the justfile)"
+    default: "dev-2026-09"
+  packages:
+    description: "Extra apt packages to install"
+    default: ""
+
+runs:
+  using: "composite"
+  steps:
+    - name: Install System Packages
+      shell: bash
+      run: |
+        SUDO=""; command -v sudo >/dev/null && SUDO=sudo
+        $SUDO apt-get update
+        # clang links Odin programs on Linux; raylib needs X11
+        $SUDO apt-get install -y --no-install-recommends \
+          clang libx11-dev curl ca-certificates unzip ${{ inputs.packages }}
+
+    - name: Install Odin
+      shell: bash
+      run: |
+        curl -fsSL -o /tmp/odin.tar.gz \
+          https://github.com/odin-lang/Odin/releases/download/${{ inputs.odin-version }}/odin-linux-amd64-${{ inputs.odin-version }}.tar.gz
+        mkdir -p "$HOME/odin"
+        tar -xzf /tmp/odin.tar.gz -C "$HOME/odin" --strip-components=1
+        echo "$HOME/odin" >> "$GITHUB_PATH"
+
+    - name: Install just
+      shell: bash
+      run: |
+        mkdir -p "$HOME/.local/bin"
+        curl --proto '=https' --tlsv1.2 -sSf https://just.systems/install.sh | bash -s -- --to "$HOME/.local/bin"
+        echo "$HOME/.local/bin" >> "$GITHUB_PATH"