the asset system is reworked to support "dynamic" entries, where each
entry is a separate file on disk containing the latest generation's
headers+raw+gzip+zstd. when calling view.regenerate, it will look for
pages that had "export const regenerate" during generation, and render
those pages using the view system, but then store the results as assets
instead of sending as a response.
pages configured as regenerable are also bundled as views, using the
non-aliasing key "page:${page.id}". this cannot alias because file
paths may not contain a colon.
the problems with the original implementation was mostly around error
handling. sources had to be tracked manually and provided to each
incremental output. the `hasArtifact` check was frequently forgotten.
this has been re-abstracted through `incr.work()`, which is given an
`io` object. all fs reads and module loads go through this interface,
which allows the sources to be properly tracked, even if it throws.
closes#12
