Add mutation testing to validate the test suite

Introduce a self-contained mutation testing harness that verifies the unit
tests actually catch bugs: it makes small deliberate breakages to the library
(flip comparisons, delete statements, swap true/false, etc.), rebuilds, and
runs the whole CTest suite against each mutant. Tests that still pass reveal a
gap; tests that fail "kill" the mutant.

- scripts/mutation_test.py: the engine (stdlib only, no LLVM/clang deps).
  Operators ROR/LCR/BCR/AOR/ICR/SDL over src/error.c and the macro header.
  Mutates a scratch copy, never the working tree. Supports --target, --list,
  --max-mutants sampling, --threshold gating, --timeout.
- CMakeLists.txt: 'mutation' custom target (cmake --build build --target mutation).
- .gitea/workflows/ci.yaml: gated mutation job on src/error.c (threshold 65%).
- tests/MUTATION.md: how to run, interpret survivors, and known equivalents.

Close the real gaps the harness found in src/error.c (score 53% -> 71%):
- err_error_names: the AKERR_* codes have their names registered by akerr_init
- err_release_clears: releasing a context wipes it before reuse
- err_pool_exhaust: akerr_next_error returns NULL when the pool is full and
  always hands back the lowest free slot

Also surfaced (documented, not fixed): AKERR_MAX_ERR_VALUE (+15) is below
AKERR_NOT_IMPLEMENTED (+16) and AKERR_BADEXC (+17), so those codes can never
have a name registered.

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

tests/err_release_clears.c

@@ -0,0 +1,44 @@
+#include "akerror.h"
+#include "err_capture.h"
+#include <string.h>
+
+/*
+ * Releasing an error context back to the pool must wipe it, so the next caller
+ * that checks it out never sees stale status/message/stacktrace from a previous
+ * error. Mutation testing showed the clearing memset in akerr_release_error
+ * could be deleted without any test noticing.
+ */
+
+akerr_ErrorContext *boom(void)
+{
+    PREPARE_ERROR(e);
+    FAIL_RETURN(e, AKERR_VALUE, "stale dirty message that must not survive");
+}
+
+int main(void)
+{
+    akerr_capture_install();
+    akerr_init();
+
+    /* Raise and fully handle an error; FINISH_NORETURN releases it to the pool. */
+    PREPARE_ERROR(e);
+    ATTEMPT {
+        CATCH(e, boom());
+    } CLEANUP {
+    } PROCESS(e) {
+    } HANDLE(e, AKERR_VALUE) {
+    } FINISH_NORETURN(e);
+
+    AKERR_CHECK(e == NULL);
+
+    /* The next context handed out is the slot we just released: it must be clean. */
+    akerr_ErrorContext *slot = akerr_next_error();
+    AKERR_CHECK(slot != NULL);
+    AKERR_CHECK(slot->status == 0);
+    AKERR_CHECK(slot->message[0] == '\0');
+    AKERR_CHECK(slot->stacktracebuf[0] == '\0');
+    AKERR_CHECK(strstr(slot->message, "stale dirty message") == NULL);
+
+    fprintf(stderr, "err_release_clears ok\n");
+    return 0;
+}