Add collision-safe status code registry

Replace the consumer-sized status-name array with private sparse storage and accept arbitrary integer status values. Add explicit range reservations with overlap diagnostics, reserve the library's 0-255 compatibility band, and harden pointer and string boundary handling.

Update regression coverage and document the required migration for custom status-code consumers.

tests/err_name_bounds.c

@@ -2,13 +2,7 @@
 #include "err_capture.h"
 #include <string.h>
 
-/*
- * akerr_name_for_status must reject out-of-range status codes at BOTH ends.
- * It already guarded the upper bound, but a negative status indexed
- * __AKERR_ERROR_NAMES[negative] -- an out-of-bounds read (and an out-of-bounds
- * write when a name is supplied). Every out-of-range status must return the
- * "Unknown Error" sentinel instead.
- */
+/* Unregistered status values return the sentinel regardless of magnitude. */
 
 int main(void)
 {
@@ -18,8 +12,7 @@ int main(void)
     AKERR_CHECK(strcmp(akerr_name_for_status(-1, NULL), "Unknown Error") == 0);
     AKERR_CHECK(strcmp(akerr_name_for_status(-9999, NULL), "Unknown Error") == 0);
 
-    /* Above range (already handled; kept as a guard against regressions). */
-    AKERR_CHECK(strcmp(akerr_name_for_status(AKERR_MAX_ERR_VALUE + 1, NULL),
+    AKERR_CHECK(strcmp(akerr_name_for_status(1000000, NULL),
 		       "Unknown Error") == 0);
 
     /* A valid code must still resolve to its real name. */