- Introduce SeekMode::Rope for absolute rope-index positioning
- Add CursorState.offset field to support local coordinate systems per cursor
- Implement ForwardCursor.cursor() and BackwardCursor(cursor()) to create sub-cursors with independent offsets
- Update tell(), get(i), set i, len() to use local coordinates (relative offset)
- Add rope_tell(), reset()—deprecate old absolute behavior in favor of offset-aware API
- Add comprehensive tests for sub-cursor semantics, including write/reset and bounds checking