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>

include/akerror.tmpl.h

@@ -40,7 +40,10 @@
 #define AKERR_BADEXC                (AKERR_LAST_ERRNO_VALUE + 17)  /** The libakerr library was given an akerr_ErrorContext to parse that did not come from AKERR_ARRAY_ERROR (likely an uninitialized pointer) */
 
 #ifndef AKERR_MAX_ERR_VALUE
-#define AKERR_MAX_ERR_VALUE (AKERR_LAST_ERRNO_VALUE + 15)
+/* Must be >= the highest AKERR_* offset above (AKERR_BADEXC, +17) so every
+ * library error code is indexable in __AKERR_ERROR_NAMES. Keep in sync when
+ * adding codes; tests/err_maxval.c guards this invariant. */
+#define AKERR_MAX_ERR_VALUE (AKERR_LAST_ERRNO_VALUE + 17)
 #elif AKERR_MAX_ERR_VALUE < 256
 #error user-defined AKERR_MAX_ERR_VALUE must be >= 256
 #endif