Summary

• join: Join lines of two sorted files on common field (-1, -2, -t, -a, -e)
• split: Split files into pieces by lines (-l), bytes (-b), or chunks (-n), with -d for numeric suffixes

Test plan

• 5 join tests: basic, custom field, custom separator, missing operand, unpairable lines
• 8 split tests: by lines, numeric suffix, by chunks, custom prefix, missing file, zero chunks, suffix generation
• clippy clean, 1653 tests pass

Closes #547, Closes #543