Add code coverage job to CI

Author: colemancda

.github/workflows/ci.yml

@@ -18,6 +18,36 @@ jobs:
       - name: Test
         run: swift test -v
 
+  coverage:
+    name: Code Coverage
+    runs-on: macos-26
+    permissions:
+      contents: read
+      code-quality: write # required by actions/upload-code-coverage
+    steps:
+      - uses: actions/checkout@v4
+      - name: Export coverage and enforce threshold
+        run: Scripts/coverage.sh
+        env:
+          COVERAGE_THRESHOLD: 65
+      - name: Upload coverage to GitHub Code Quality
+        uses: actions/upload-code-coverage@v1
+        with:
+          file: .build/coverage/coverage.xml
+          language: Swift
+          label: code-coverage/llvm-cov
+          # Code Quality was in public preview until 2026-07-20; keep this while
+          # the repo might not have the feature enabled, so a failed upload
+          # doesn't break CI.
+          fail-on-error: false
+      - name: Upload coverage artifacts
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: code-coverage
+          path: .build/coverage/
+          if-no-files-found: error
+
   linux:
     runs-on: ubuntu-latest
     container: swift:6.3.3