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>

tests/MUTATION.md

@@ -42,7 +42,20 @@ cmake --build build --target mutation
 
 Useful flags: `--timeout SECONDS` (per-suite build+test cap; a mutant that
 hangs is counted as killed), `--keep` (retain the scratch copy for debugging),
-`--work DIR` (use a specific scratch directory).
+`--work DIR` (use a specific scratch directory), `--junit FILE` (write a JUnit
+XML report — surviving mutants appear as failing test cases).
+
+## CI reporting
+
+Both the unit tests and the mutation run emit JUnit XML that CI consumes:
+
+* `ctest --test-dir build --output-junit "$(pwd)/ctest-junit.xml"` — note the
+  absolute path; `--output-junit` otherwise resolves relative to the test dir.
+* `scripts/mutation_test.py --junit mutation-junit.xml`
+
+`.gitea/workflows/ci.yaml` runs both and feeds the XML to
+`mikepenz/action-junit-report` (with `if: always()`, so results publish even
+when a gate fails). The generated `*-junit.xml` files are git-ignored.
 
 ## Mutation operators