Add `InputBuffer` with `push_char`/`push_str`/`pop_char`/`clear` tracking
cursor position in display columns (CJK = 2, ASCII = 1) plus 6 unit tests
(p9-fb-10 Task 1). Re-export from crate root.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>