Compute longest distinct substring, case-insensitive

Given a string s, return the length of the longest contiguous substring that contains no repeated characters, treating letters case-insensitively (e.g., 'A' and 'a' are the same). All ASCII symbols and whitespace should be considered valid characters. Provide an algorithm with optimal time complexity, justify its time and space bounds, and discuss edge cases (empty string, all duplicates, Unicode input). For extremely large inputs that cannot fully fit in memory, outline how you would process the stream to compute the answer and the trade-offs involved.