Build `time_travel_refactor(old_code, modernize)` — a mock refactor pass that replaces 1998-era idiom keys with 2026 ones: - `old_code` is a string of Python source. - `modernize` is a dict mapping old-token → new-token (e.g. `{"format(": "f-string("}`). - Replace every occurrence of each key with its value, in iteration order of the dict. - Empty `modernize` dict → return `old_code` unchanged. - Empty `old_code` → return `""`. This is the algorithmic core of an autofix pass — the real tool wraps an AST visitor around the same idea.