Add Spinner, a Minecraft hostname-routing reverse proxy

Routes Java Edition connections by the hostname in the handshake, with
wildcard domains, backend failover, PROXY protocol v2, cached server-list
status from health probes, hot-reloaded config, and an admin page and
metrics endpoint. Includes CI, tagged releases and a container image.

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

.gitea/workflows/ci.yml

@@ -0,0 +1,53 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    env:
+      GOPRIVATE: git.veillax.com
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+
+      # Go fetches Reef straight from this server. This is harmless for public
+      # repositories and is what makes private ones work.
+      - name: Let Go fetch modules from this server
+        run: git config --global url."https://x-access-token:${{ secrets.GITHUB_TOKEN }}@git.veillax.com/".insteadOf "https://git.veillax.com/"
+
+      - name: Formatting
+        run: |
+          unformatted=$(gofmt -l .)
+          if [ -n "$unformatted" ]; then
+            echo "These files need gofmt:"
+            echo "$unformatted"
+            exit 1
+          fi
+
+      - name: Shell scripts parse
+        run: |
+          for f in deploy/*.sh; do sh -n "$f"; done
+
+      - name: Design language
+        run: node scripts/lint-design.mjs
+
+      - name: Vet
+        run: go vet ./...
+
+      - name: Test
+        run: go test ./...
+
+      # Catches platform-specific breakage before a release is tagged.
+      - name: Every release platform compiles
+        run: |
+          for t in linux/amd64 linux/arm64 windows/amd64; do
+            echo "building for $t"
+            GOOS=${t%/*} GOARCH=${t#*/} CGO_ENABLED=0 go build -o /dev/null ./cmd/spinner
+          done