Step Pattern

The sequencer/arpeggiator pattern model and the grid it draws on, as pure maths: held notes walked up, down, up-down (endpoints unrepeated), in press order or by a seeded per-step hash — transcribed from the shipped arpeggiator and pinned bit-identical against it — plus octave stacking, pattern rotation, a sequence-to-lanes resolver for read-only piano-roll views, page arithmetic, an exact steps × lanes tiling with inverse hit-testing, and the tap/paint/velocity-drag gesture maths. No React, no DOM — testable in Node and safe to lay out on the server.