Every span row (except the current view root) gets a hover focus control
that re-roots the waterfall at that span: the subtree is re-based to
depth 0 and duration/counts/open state are recomputed over the subtree
only, so e.g. a 2-minute $session-replay nested in a year-long
$refresh-token becomes fully readable instead of a sliver. A breadcrumb
in the header climbs back to any ancestor. Focus resets when selecting
another trace.