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.

CMakeLists.txt

@@ -125,11 +125,6 @@ foreach(_test IN LISTS AKERR_TESTS)
     add_test(NAME ${_test} COMMAND test_${_test})
 endforeach()
 
-# err_maxval parses the generated header to check AKERR_MAX_ERR_VALUE covers
-# every AKERR_* code, so it needs the path to the header it was built against.
-target_compile_definitions(test_err_maxval PRIVATE
-    AKERR_GENERATED_HEADER="${GENERATED_AKERROR_H}")
-
 set_tests_properties(
     ${AKERR_WILL_FAIL_TESTS}
     PROPERTIES WILL_FAIL TRUE