Split conversation pane into 2-char gutter + text column. Gutter shows
● markers at turn boundaries (Cyan for user, Magenta for assistant),
aligned with the input area's ' > ' gutter.
Key changes:
- Added Marker enum (None/User/Assistant) and parallel markers vec
- Track turn boundaries via pending_marker field
- New draw_conversation_pane() with visual row computation for wrapping
- Both gutter and text scroll synchronously by visual line offset
This fixes the wrapping alignment issue where continuation lines
aligned under markers instead of under the text.