feat: add xiter sub-packages and update project direction - Add xiter/io: lazy I/O (Lines, ReadDir, ReadFileByChunk) - Add xiter/stream: concurrent processing (ParallelMap, FanIn, Batch) - Add xiter/rt: time-based sequences (Ticker, Throttle, Debounce) - Add xiter/collect: grouping and windowing (GroupBy, SortedGroupBy, Window) - Add xiter/combin: combinatorial generators (Combinations, Permutations, Product) - Update README with project philosophy: lazy Seq as core direction#18