Full-viewport chapter bands; inside each, the thought-line runs HORIZONTALLY while the chapter title holds sticky (the decadree mechanic rotated 90°). Between chapters the line drops visibly to the next station. The boldest of my four.
1. This is his sticky-vote taken seriously as ARCHITECTURE: the chapter title physically holds while its evidence travels.
2. Horizontal movement is explicit (visible cue, buttons, keyboard) per the a11y law; never un-signposted (The Fold's documented failure).
3. The line's drop between stations is the page's one authored transition: the thought moving on.
4. Programme knots ON the line answers the seven-family problem inside the spine metaphor.
5. Mark-and-return: marked lines clip to the station's edge and ride the drop to the tray.
6. Risk: the highest build complexity of my four; horizontal sections are where v1 pages died in review. Chosen only if we commit to doing it properly.
7. Mobile abandons horizontality entirely: honest vertical stations, spine at left edge.