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_pool_exhaust.c

@@ -0,0 +1,39 @@
+#include "akerror.h"
+#include "err_capture.h"
+
+/*
+ * The error pool is a fixed array of AKERR_MAX_ARRAY_ERROR slots. When every
+ * slot is checked out, akerr_next_error() must return NULL rather than run off
+ * the end of the array; and it must always hand back the lowest free slot.
+ * Mutation testing showed both the terminating "return NULL" and the scan
+ * bounds could be broken without any test noticing.
+ */
+
+int main(void)
+{
+    akerr_init();
+
+    akerr_ErrorContext *slots[AKERR_MAX_ARRAY_ERROR];
+
+    /* Check out every slot. */
+    for ( int i = 0; i < AKERR_MAX_ARRAY_ERROR; i++ ) {
+        slots[i] = akerr_next_error();
+        AKERR_CHECK(slots[i] != NULL);
+        slots[i]->refcount = 1;
+    }
+
+    /* Pool is fully exhausted: the next request must fail cleanly. */
+    AKERR_CHECK(akerr_next_error() == NULL);
+
+    /* Free exactly the first slot; the scan must find and return it. */
+    slots[0]->refcount = 0;
+    AKERR_CHECK(akerr_next_error() == slots[0]);
+
+    /* Tidy up. */
+    for ( int i = 0; i < AKERR_MAX_ARRAY_ERROR; i++ ) {
+        slots[i]->refcount = 0;
+    }
+
+    fprintf(stderr, "err_pool_exhaust ok\n");
+    return 0;
+}