Custom Sort String

Write a function `custom_sort_string(order: str, s: str) -> str` that returns a permutation of `s` such that the characters which appear in `order` are placed first, arranged in the same relative sequence as they appear in `order`. All characters not present in `order` should come after those, preserving their original order in `s`. The sorting must be stable: for characters that do not appear in `order`, their relative order in `s` must be maintained. Examples: - custom_sort_string("cba", "abcd") -> "cbad" (c and b come first as per order, then a, then d because d is not in order). - custom_sort_string("kqep", "pekeq") -> "kqeep" (characters in order are placed as k,q,e; remaining: e, p? Actually implement according to definition). Verify with provided examples. Implement exactly the function signature above. Do not use external libraries.

>>> custom_sort_string("cba", "abcd")
'cbad'
>>> custom_sort_string("kqep", "pekeq")
'kqeep'
>>> custom_sort_string("abc", "cba")
'cb'? Wait "cba" has a,b,c? Actually order abc, s=cba -> all characters in order, so return 'abc'? Let's see: order 'abc', s 'cba' -> expected 'abc'? That is not stable? Actually the definition: characters that appear in order are placed first in order of order; all others appended stable. Here all characters appear in order, so return 'abc'.
>>> custom_sort_string("", "abc")
'abc'