One column, phone-first. You always see two levels: the blocks of the current level, and inside each block a preview of what it contains. Tap a block to step into it; the breadcrumb takes you back up. No canvas, no panning — structure instead of scrolling in two dimensions.
Tap any block with a ↳ enter mark to open it. The phone back button also walks back up. Leave a note on anything that reads wrong — the caption of each block is the note handle.