ci: publish this build's own coverage and benchmarks

The board and detail projections in beads/ and the memory markdown render in
web/ are what a request costs here — there is no SQL engine behind a bare
store, so every table is read whole and projected in process. This repository
had no Benchmark at all; those two paths now have one each, driven through the
seams that already exist (a canned BrowseSession, an index built by the real
PrefixesAcross), so what is timed is the projection and not the disk.

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.