Don’t “compile” assets, just “build”

This reserves the `build/results/` directory for _just_ the results of
transformations, and delays the copying of asset files into the build
directory until the actual “build” step. (Likewise for recursive files,
altho these still just error).

`make all` now builds all installable files, including assets, which
were formerly excluded. A downstream script might expect assets to
appear in `build/public` after a `make all` and shouldn’t require a
`make install` to get them.