Serve a directory tree over HTTP through one embedded page: a listing
with download links and a multi-file upload form.

Filesystem access is confined to the served root with os.Root, and
only regular files are served. Uploads are bounded by size and part
count, and every file is staged in the target directory before any is
renamed into place, so a refused or interrupted request leaves the
directory untouched.

Standard library only, no third-party dependencies.