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_errno.c

@@ -6,7 +6,7 @@
  * The library imports system errno codes and their descriptions at build time
  * (scripts/generrno.sh -> akerr_init_errno). Verify:
  *   - a system errno (EACCES) has a registered, non-empty name;
- *   - an out-of-range status returns the "Unknown Error" sentinel;
+ *   - an unregistered status returns the "Unknown Error" sentinel;
  *   - a system errno can be raised, propagated and handled like any AKERR_* code.
  */
 
@@ -28,7 +28,7 @@ int main(void)
     AKERR_CHECK(nm[0] != '\0');
     AKERR_CHECK(strcmp(nm, "Unknown Error") != 0);
 
-    AKERR_CHECK(strcmp(akerr_name_for_status(AKERR_MAX_ERR_VALUE + 5000, NULL),
+    AKERR_CHECK(strcmp(akerr_name_for_status(1000000, NULL),
 		       "Unknown Error") == 0);
 
     PREPARE_ERROR(e);