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_error_names.c

@@ -7,9 +7,9 @@
  * Verify the names are actually installed (mutation testing showed the
  * registration calls could be deleted without any test noticing).
  *
- * Note: AKERR_NOT_IMPLEMENTED and AKERR_BADEXC are intentionally omitted -- they
- * exceed AKERR_MAX_ERR_VALUE, so akerr_name_for_status cannot store or return
- * their names (see tests/MUTATION.md).
+ * Note: AKERR_EOF, AKERR_ITERATOR_BREAK and AKERR_NOT_IMPLEMENTED are omitted --
+ * they are valid codes but akerr_init does not register a display name for them,
+ * so akerr_name_for_status returns an empty string rather than a known name.
  */
 
 static const struct {
@@ -28,6 +28,7 @@ static const struct {
     { AKERR_VALUE,              "Value Error" },
     { AKERR_RELATIONSHIP,       "Relationship Error" },
     { AKERR_CIRCULAR_REFERENCE, "Circular Reference Error" },
+    { AKERR_BADEXC,             "Invalid akerr_ErrorContext" },
 };
 
 int main(void)