ci: add Gitea Actions build workflow

Builds Rust + .NET on push/PR. On version tags, publishes
framework-dependent win-x64 zip + gatunad linux binary as
Gitea release assets via REST API.

Runs on hugmaster (HuggingFace Spaces runner).

.gitea/workflows/build.yml

@@ -0,0 +1,55 @@
+name: build
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: hugmaster
+    steps:
+      - name: Checkout
+        run: |
+          git clone "${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}.git" .
+          git checkout "$GITHUB_SHA"
+
+      - name: Build Rust
+        run: |
+          cd gatunad
+          cargo build --release
+
+      - name: Build .NET
+        run: |
+          cd gatuna-win
+          dotnet build
+
+      - name: Publish Windows (framework-dependent)
+        if: ${{ startsWith(github.ref, 'refs/tags/v') }}
+        run: |
+          cd gatuna-win
+          dotnet publish -c Release -r win-x64 --no-self-contained -o ../publish/win
+
+      - name: Package release assets
+        if: ${{ startsWith(github.ref, 'refs/tags/v') }}
+        run: |
+          mkdir -p release
+          cp gatunad/target/release/gatunad release/gatunad-linux-amd64
+          cd publish/win && zip -r ../../release/gatuna-win.zip . && cd ../..
+
+      - name: Create Gitea release
+        if: ${{ startsWith(github.ref, 'refs/tags/v') }}
+        run: |
+          TAG="${GITHUB_REF#refs/tags/}"
+          RESP=$(curl -sS -X POST \
+            -H "Authorization: token ${GITHUB_TOKEN}" \
+            -H "Content-Type: application/json" \
+            -d "{\"tag_name\":\"$TAG\",\"name\":\"$TAG\",\"body\":\"Automated build for $TAG\"}" \
+            "${GITHUB_SERVER_URL}/api/v1/repos/${GITHUB_REPOSITORY}/releases")
+          RID=$(echo "$RESP" | jq .id)
+          curl -sS -X POST \
+            -H "Authorization: token ${GITHUB_TOKEN}" \
+            -H "Content-Type: application/octet-stream" \
+            --data-binary @release/gatunad-linux-amd64 \
+            "${GITHUB_SERVER_URL}/api/v1/repos/${GITHUB_REPOSITORY}/releases/${RID}/assets?name=gatunad-linux-amd64"
+          curl -sS -X POST \
+            -H "Authorization: token ${GITHUB_TOKEN}" \
+            -H "Content-Type: application/octet-stream" \
+            --data-binary @release/gatuna-win.zip \
+            "${GITHUB_SERVER_URL}/api/v1/repos/${GITHUB_REPOSITORY}/releases/${RID}/assets?name=gatuna-win.zip"