Implement check_password(pw, username="") returning a list of all failed rules (empty if valid), using these exact messages in this order:
"at least 12 characters""a lowercase letter""an uppercase letter""a digit""a symbol" (any character that isn't a letter, digit or whitespace)"no spaces" (fails if it contains any whitespace)"must not contain the username" (case-insensitive; only checked if username is non-empty)