Record the mutation score and the concurrency mutants it misses

src/error.c now scores 81.2%: 238 of 293 mutants killed, 204 by a
failing test, 24 by failing to compile, and 10 by hanging the suite --
deleting akerr_mutex_init() or the akerr_initializing re-entry guard
deadlocks the first test, which is the right answer for a broken lock.

Lock deletions are the one survivor category where surviving does not
mean harmless, so measure it rather than assume: rebuilt, the surviving
"delete the pool lock" mutant fails tests/err_threads_pool.c in 4 of 10
runs and fails under scripts/thread_test.sh in 5 of 5, with no false
positive on the unmutated library. The property assertions alone are a
coin flip on a missing lock; the sanitizer run is what holds that line.
The harness builds mutants with default CMake options and so never sees
it -- TODO item 8.

Also warn that a sanitized test binary run by hand does not inherit the
halt_on_error CTest gives it, and will print a race and still exit 0.
That is how the 5-of-5 above first read as 2 of 5.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

AGENTS.md

@@ -40,8 +40,11 @@ which configures `build/tsan` with `-DAKERR_SANITIZE=thread`, builds the library
 *and* the tests with it, and runs CTest with ASLR disabled (TSan aborts with an
 "unexpected memory mapping" on kernels with `vm.mmap_rnd_bits` above 28).
 `AKERR_SANITIZE` takes any sanitizer list, so `-DAKERR_SANITIZE=address,undefined`
-works the same way. Under a sanitized build a report fails the test rather than
-being printed and passed over.
+works the same way. CTest gives each test `halt_on_error=1`, so a report fails
+the test rather than being printed and passed over — running a sanitized test
+binary by hand does **not** inherit that. Set it yourself
+(`TSAN_OPTIONS=halt_on_error=1 ./build/tsan/test_err_threads_pool`) or the
+binary can print a race and still exit 0.
 
 Mutation testing is available through: