Combine a list of configuration dictionaries from left to right, with later layers taking precedence. When an existing value and an incoming value are both dictionaries, merge them recursively. In every other case, replace the old value completely with the incoming value, including lists, False, zero, and None. A list is replaced, not concatenated. None is an explicit value, not a deletion marker. Return {} for no layers. All data is JSON-compatible, and nested dictionaries have string keys. Do not mutate any layer. Grading checks values and input integrity, not object identity.