Fix AKERR_MAX_ERR_VALUE to cover all AKERR_* codes

AKERR_MAX_ERR_VALUE was AKERR_LAST_ERRNO_VALUE + 15, but the highest defined
code, AKERR_BADEXC, is + 17 (AKERR_NOT_IMPLEMENTED is + 16). akerr_name_for_status
rejects any status above the max, so those codes could never have a registered
name and the AKERR_BADEXC registration in akerr_init was dead code -- a gap
found by mutation testing. Bump the max to + 17.

- err_maxval: new test asserting the reserved AKERR_* range exceeds the number
  of AKERR_* codes and that every code is individually indexable. Fails against
  the old + 15 value (verified), guarding against regression.
- err_error_names: now also checks AKERR_BADEXC's name, which the fix makes
  reachable.

Mutation score on src/error.c rises 71% -> 74%: the previously-dead BADEXC
registration and the name_for_status upper-bound check are now killable.

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

tests/err_maxval.c

@@ -0,0 +1,52 @@
+#include "akerror.h"
+#include "err_capture.h"
+
+/*
+ * AKERR_MAX_ERR_VALUE sizes the __AKERR_ERROR_NAMES table and is the upper bound
+ * akerr_name_for_status() will accept. If it is smaller than the highest AKERR_*
+ * code, those codes silently lose their names (this was a real bug: the max was
+ * +15 while AKERR_BADEXC is +17).
+ *
+ * Guard the invariant: the AKERR_* range reserved above AKERR_LAST_ERRNO_VALUE
+ * must be larger than the number of AKERR_* codes defined, and every code must
+ * be individually indexable.
+ */
+
+/* Every AKERR_* library error code. Keep in sync with akerror.h. */
+static const int akerr_codes[] = {
+    AKERR_NULLPOINTER,
+    AKERR_OUTOFBOUNDS,
+    AKERR_API,
+    AKERR_ATTRIBUTE,
+    AKERR_TYPE,
+    AKERR_KEY,
+    AKERR_INDEX,
+    AKERR_FORMAT,
+    AKERR_IO,
+    AKERR_VALUE,
+    AKERR_RELATIONSHIP,
+    AKERR_EOF,
+    AKERR_CIRCULAR_REFERENCE,
+    AKERR_ITERATOR_BREAK,
+    AKERR_NOT_IMPLEMENTED,
+    AKERR_BADEXC,
+};
+
+int main(void)
+{
+    int n = (int)(sizeof(akerr_codes) / sizeof(akerr_codes[0]));
+
+    /* AKERR_MAX_ERR_VALUE must reserve more codes than are actually defined. */
+    AKERR_CHECK((AKERR_MAX_ERR_VALUE - AKERR_LAST_ERRNO_VALUE) > n);
+
+    /* Stronger: every defined code must fall within the addressable range so
+     * its name can be stored and retrieved. */
+    for ( int i = 0; i < n; i++ ) {
+        AKERR_CHECK(akerr_codes[i] > AKERR_LAST_ERRNO_VALUE);
+        AKERR_CHECK(akerr_codes[i] <= AKERR_MAX_ERR_VALUE);
+    }
+
+    fprintf(stderr, "err_maxval ok (%d codes, range %d)\n",
+	    n, AKERR_MAX_ERR_VALUE - AKERR_LAST_ERRNO_VALUE);
+    return 0;
+}