The Braid and Compact switches become a single picker: Original (Haven's
own layout) and every <Name>Layout.plugin.js on the server, one at a time.
A new layout plugin shows up without any change to Haven, so alternative
sidebars can ship as layouts while Original stays the default. The picker
shows the chosen layout even while it waits (Compact needs a wide window,
Braid waits for Mod Mode), and follows Braid's own shortcut and menu.
theme-authoring.md says what a plugin needs to appear there.
ancsemi