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>

.gitignore

@@ -1,2 +1,3 @@
 include/akerror.h
 build/
+*-junit.xml