ci: publish this build's own coverage and benchmarks

Serving a revision is two passes over the whole corpus — FromDocuments builds
the archive, LinkPass renders every document to fill the link graph — and a
proposal view is a third over two whole revisions. All three grow with the
space rather than with the request, and this repository had no Benchmark at
all; doc/ and prosediff/ now measure them over a synthetic 208-document space
and a 24-chapter specification carrying one of every change kind.

The manifest gains a coverage and a bench task, both after publish so a
rejected upload cannot cost a shipped apk, and both guarded: the profile must
be non-empty, the benchmark names must be in the file (an empty benchfmt body
uploads and reports success), and a build handed no token says so and exits 0
with the artifact still attached.