Add coverage reporting to CI

swift test runs with --enable-code-coverage and each matrix job prints
an llvm-cov per-file + total summary (informational only - no
thresholds, so coverage can never be the reason a build goes red).
Local baseline at introduction: 69.7% lines.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/swift-tests.yml

@@ -39,7 +39,23 @@ jobs:
             ${{ runner.os }}-${{ matrix.name }}-
 
       - name: Run Tests
-        run: swift test --parallel --quiet --package-path ${{ matrix.path }}
+        run: swift test --parallel --quiet --enable-code-coverage --package-path ${{ matrix.path }}
+
+      # Informational only: surfaces per-file and total line coverage in the
+      # job log so coverage trends are visible on every PR. No thresholds —
+      # this must never be the reason a build goes red.
+      - name: Coverage summary
+        run: |
+          BIN_PATH=$(swift build --show-bin-path --package-path ${{ matrix.path }})
+          PROF="$BIN_PATH/codecov/default.profdata"
+          XCTEST=$(find "$BIN_PATH" -maxdepth 1 -name '*.xctest' | head -1)
+          BINARY="$XCTEST/Contents/MacOS/$(basename "$XCTEST" .xctest)"
+          if [ -f "$PROF" ] && [ -f "$BINARY" ]; then
+            xcrun llvm-cov report "$BINARY" -instr-profile "$PROF" \
+              -ignore-filename-regex='(Tests|\.build|checkouts|Mocks|_PreviewHelpers)' || true
+          else
+            echo "No coverage data found; skipping summary."
+          fi
 
   # SPM tests above only compile the macOS slice; this job covers the
   # iOS-conditional code paths (UIKit, CoreBluetooth restoration, etc.).