Explorer was firing temporal requests before the graph even loaded.
When the backend is down, /api/graph/nodes fails but the temporal
bounds and snapshot effects didn't care , they fired anyway, off in
their own corner, ignoring whether the graph actually came up. Every
page load with no backend meant three failed requests instead of one,
and a scrubber that had nothing to scrub.
Added two small predicate functions and gated the temporal effects on
them. Basically: don't ask for time-based data until you know the
graph itself loaded. An empty graph still counts as loaded, so that
case isn't broken.
Confirmed with the backend down, before and after: three failing
requests down to one.
Fixes#982.
