ci: add test pipeline with codecov, staticcheck, and race detection (#10)

* ci: add test pipeline with codecov, staticcheck, and race detection

- Build/test matrix: Go 1.25 + 1.26
- Race detection enabled for all tests
- Coverage uploaded to Codecov (latest Go only)
- staticcheck lint step
- Go module caching via setup-go

* ci: drop Go 1.25 from matrix (go.mod requires 1.26)

.github/workflows/ci.yml

@@ -0,0 +1,55 @@
+name: CI
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    name: Test (Go ${{ matrix.go-version }})
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        go-version: ["1.26"]
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Go ${{ matrix.go-version }}
+        uses: actions/setup-go@v5
+        with:
+          go-version: ${{ matrix.go-version }}
+          cache: true
+
+      - name: Run tests with race detection and coverage
+        run: go test -race -coverprofile=coverage.out -covermode=atomic ./...
+
+      - name: Upload coverage to Codecov
+        if: matrix.go-version == '1.26'
+        uses: codecov/codecov-action@v5
+        with:
+          files: coverage.out
+          token: ${{ secrets.CODECOV_TOKEN }}
+          fail_ci_if_error: false
+
+  lint:
+    name: Lint
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: "1.26"
+          cache: true
+
+      - name: Run staticcheck
+        uses: dominikh/staticcheck-action@v1
+        with:
+          version: latest
+          install-go: false