ci: add GitHub Actions workflow

.github/workflows/ci.yml

@@ -0,0 +1,43 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+
+      - name: Build
+        run: go build ./...
+
+      - name: Vet
+        run: go vet ./...
+
+      # -short skips the slow geo/TopoJSON specs (~2-40s each); the rest of
+      # the 600+-spec regression corpus still runs. Everything needed is
+      # committed (vendored JS modules, WASM binaries, test data), so no
+      # network access is required.
+      - name: Test
+        run: go test -short ./...
+
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+
+      - uses: golangci/golangci-lint-action@v8