Sprites JavaScript Sequencer Media

The sequencer media: a tracker-style editor that drives a platform's native sound channels. There is no canvas; the editor is three stacked areas, a horizontally scrolling strip of block slots at the top, a vertically scrolling grid of beats by channels in the middle, and a piano keyboard at the bottom, with floating windows for the note, the info and the transport. The core module holds the state, the editor and its document helpers; each area and window is a module of its own under Sprites.Media.Sequencer. Every channel's capabilities come from the platform's Sprites.Platform.<name>.Sequencer descriptor, so one editor drives them all. These pages are a skeleton ahead of the build.