build: add test report generator (task 0003)

Add scripts/test-report.ts (bun run test:report): it runs the full suite
with JUnit + lcov output, then renders a gitignored reports/test-report.md
— a summary, a describe-grouped inventory (pass/todo/fail), and a
coverage table. The report is written even when tests fail and the script
exits with the test run's own status, so CI can gate on it later. Adds
fast-xml-parser (dev) for robust JUnit parsing; lcov is parsed by hand.

.gitignore

@@ -10,6 +10,9 @@ result-*
 # test coverage output
 coverage/
 
+# generated test report and its intermediate artifacts
+reports/
+
 # direnv / nix-direnv cache
 .direnv/