The spine runs down the CENTRE; thought alternates left and right of it like a considered conversation, and the sketches sit in the gutter crossing the line. A metronome reading rhythm nobody in v1 tried.
1. Alternation enacts "thinking with someone": statement one side, reframe the other; the gutter is the table between them.
2. Sketches CROSS the gutter (half left, half right): literally the thought bridging to a next move.
3. The programme detail opens ACROSS the gutter, temporarily unifying the columns: choosing a programme unites the conversation.
4. Ground event stays a single inverted chapter block on the right; the line ignites through it.
5. Mark-and-return: marks slide INTO the gutter as ticks and travel down to the tray.
6. Risk: centre-spine layouts can waste side whitespace at wide viewports; the alternation must be composed, not mirrored mechanically.
7. Mobile: the gutter docks left (thin), alternation becomes an indent rhythm; nothing hides.