Emit JUnit XML from tests + mutation, consume it in CI

Produce machine-readable results and surface them in the Gitea pipeline:

- ctest: run with --output-junit to write ctest-junit.xml. The path must be
  absolute ("$(pwd)/...") because --output-junit otherwise resolves relative to
  the --test-dir build directory.
- mutation_test.py: new --junit FILE option writes a JUnit report where each
  mutant is a test case and a surviving mutant is a <failure> (so gaps show up
  as failing tests).
- .gitea/workflows/ci.yaml: both jobs generate their XML and feed it to
  mikepenz/action-junit-report with `if: always()`, so results publish even
  when a gate fails. Mutation publishing is display-only (fail_on_failure:
  false); the --threshold flag remains the gate.
- .gitignore: ignore the generated *-junit.xml artifacts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yaml

@@ -13,13 +13,23 @@ jobs:
         run: |
           sudo apt-get update -y
           sudo apt-get install -y cmake gcc moreutils
-      - name: build and test
+      - name: build and install
         run: |
           mkdir installdir
           cmake -S . -B build -DCMAKE_INSTALL_PREFIX=installdir
           cmake --build build
           cmake --install build
-          cmake --build build --target test
+      # --output-junit resolves relative to the test dir, so give an absolute
+      # path to land the report in the workspace root for the reporter below.
+      - name: test (JUnit)
+        run: ctest --test-dir build --output-on-failure --output-junit "$(pwd)/ctest-junit.xml"
+      - name: publish test results
+        if: always()
+        uses: mikepenz/action-junit-report@v4
+        with:
+          report_paths: 'ctest-junit.xml'
+          check_name: 'ctest results'
+          fail_on_failure: 'true'
       - run: echo "🍏 This job's status is ${{ job.status }}."
 
   mutation_test:
@@ -38,5 +48,14 @@ jobs:
       # deeper (slower) coverage including the macro header.
       - name: mutation testing
         run: |
-          python3 scripts/mutation_test.py --target src/error.c --threshold 65
+          python3 scripts/mutation_test.py --target src/error.c --junit mutation-junit.xml --threshold 65
+      # Publish even when the threshold gate fails, so survivors are visible.
+      # Display-only (fail_on_failure: false); the --threshold above is the gate.
+      - name: publish mutation results
+        if: always()
+        uses: mikepenz/action-junit-report@v4
+        with:
+          report_paths: 'mutation-junit.xml'
+          check_name: 'mutation results'
+          fail_on_failure: 'false'
       - run: echo "🍏 This job's status is ${{ job.status }}."