Keep the First Occurrence of Every Globally Unique Line

Implement global line uniqueness: keep only the first occurrence of each distinct input line, even when duplicates are not adjacent. ### Function Signature `unique_lines(lines: list[str]) -> list[str]` ### Rules - Compare complete strings exactly and case-sensitively. - Preserve the input order of first occurrences. - An empty string is a valid line and may appear once in the output. - Do not trim spaces or normalize the contents. - Input strings represent lines without their terminating newline characters. These comparison and order rules are explicit conventions for the global-uniqueness task. ### Constraints - `0 <= len(lines) <= 200000`. - Each line contains printable ASCII characters with code points 32 through 126, or is empty. - Total input character count is at most 2000000. - Do not mutate the input. ### Examples Input: `lines = ["alpha","beta","alpha","beta","gamma"]` Output: `["alpha","beta","gamma"]` Input: `lines = ["","A","a",""," A","A"]` Output: `["","A","a"," A"]` Input: `lines = []` Output: `[]`