Raise errors from the status registry instead of returning codes

akerr_reserve_status_range() and akerr_register_status_name() returned
private int enumerations, which was the one place in the library where a
failure was not an akerr_ErrorContext *. They now return one like
everything else: NULL on success, and on refusal an error whose status is
a real code in the library's reserved band, so it can be CATCH-ed,
HANDLE-d, PASS-ed, or left to propagate into a stack trace. Both are
marked AKERR_NOIGNORE, so discarding the result warns at compile time.

AKERR_STATUS_RANGE_OK and AKERR_STATUS_NAME_OK are gone; the remaining
seven codes move into the AKERR_* offset span and get registered names.
AKERR_LAST_LIBRARY_STATUS replaces AKERR_BADEXC as the top of that span
in the reserved-band static assert and the exhaustiveness sweep.

The refusal detail that used to go straight to akerr_log_method now
travels in the error message, so a caller that handles the error decides
whether it is reported. The two-argument akerr_name_for_status() set path
is the exception: it returns a name and cannot raise, so it logs and
releases. akerr_init() likewise has no caller to raise into, so failing
to reserve its own band or name its own codes is logged and fatal --
that can only happen on a misconfigured build, and continuing would
degrade every later stack trace to "Unknown Error".

Move the 1.0.0 upgrade notice out of README.md into UPGRADING.md and
rewrite its return-code tables in terms of the statuses now raised.

Tests: ctest 29/29, coverage 97.5% line / 64.5% branch, mutation 77.5%
(was 77.6%; the new survivors are the fatal init path, which needs a
library built with an undersized name table -- TODO item 7).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/err_error_names.c

@@ -35,6 +35,13 @@ static const struct {
     { AKERR_ITERATOR_BREAK,     "Iterator Break" },
     { AKERR_NOT_IMPLEMENTED,    "Not Implemented" },
     { AKERR_BADEXC,             "Invalid akerr_ErrorContext" },
+    { AKERR_STATUS_RANGE_OVERLAP,   "Status Range Overlap" },
+    { AKERR_STATUS_RANGE_FULL,      "Status Range Table Full" },
+    { AKERR_STATUS_RANGE_INVALID,   "Invalid Status Range" },
+    { AKERR_STATUS_NAME_UNRESERVED, "Unreserved Status Name" },
+    { AKERR_STATUS_NAME_FOREIGN,    "Foreign Status Name" },
+    { AKERR_STATUS_NAME_FULL,       "Status Name Registry Full" },
+    { AKERR_STATUS_NAME_INVALID,    "Invalid Status Name" },
 };
 
 int main(void)
@@ -52,7 +59,9 @@ int main(void)
      * AKERR_LAST_ERRNO_VALUE + 7 is the one deliberate hole (a removed code);
      * anything else nameless is a code someone added without registering it.
      */
-    for ( int offset = 1; offset <= AKERR_BADEXC - AKERR_LAST_ERRNO_VALUE; offset++ ) {
+    for ( int offset = 1;
+	  offset <= AKERR_LAST_LIBRARY_STATUS - AKERR_LAST_ERRNO_VALUE;
+	  offset++ ) {
         int code = AKERR_LAST_ERRNO_VALUE + offset;
         char *nm = akerr_name_for_status(code, NULL);
         if ( offset == 7 ) {
@@ -67,7 +76,7 @@ int main(void)
     }
 
     /* Every AKERR_* code must sit inside the band the library reserves. */
-    AKERR_CHECK(AKERR_BADEXC < AKERR_FIRST_CONSUMER_STATUS);
+    AKERR_CHECK(AKERR_LAST_LIBRARY_STATUS < AKERR_FIRST_CONSUMER_STATUS);
 
     fprintf(stderr, "err_error_names ok\n");
     return 0;