ui: add critical path measure to the Stitched Flamegraph

Stitches slices through the scheduler's critical path (from
sched.thread_executing_span): downstream, below the slice each sleep of
the selected threads happened in, the slices of the threads running on
its critical path; upstream, the threads blocked on the selected threads
with the selected slices which ran on their critical path. Weighted by
time on the critical path.

Only the critical path frames overlapping the selection are flattened,
and only for the roots whose frames overlap. Blocker slices are merged
by (thread, name path) below the node they hang below, as the flamegraph
would, before building it: a busy server blocking many clients otherwise
yields millions of nodes.