0.16.0

With more than one mode, every barrel names its mode, and none sits in the
global stylesheet.

Changed

• theme.<mode>.css for every mode, once there are two. init kept
  theme.css for the first mode and left it wired into the global stylesheet,
  so a second mode's routes loaded the first mode's tokens globally and their
  own from their layout, and got whichever came last. Now each barrel names its
  mode, the global stylesheet keeps only what every route shares (Tailwind and
  utilities.css), and each route's layout imports its own barrel. A project
  with one mode is unchanged: theme.css, wired globally.
• A project gaining a second mode is migrated. An unedited theme.css
  becomes theme.<first mode>.css, and every stylesheet import that resolves
  to it is removed, found by where it points rather than by guessing the file.
  An edited one is left, with a note on where its edits belong. check names a
  theme.css left beside two modes.