group_terminal checked only that every child was terminal, never the group
node's own state — so an empty group whose node is still Running returned
true (empty .all()), making a running node that has yet to append its
subgraph look already-finished. Now it requires the group node itself
terminal AND every child recursively terminal. Deciding when to settle a
group node to terminal once its children are done stays a scheduler concern;
this answers the dependents' question — is the whole group, node included,
finished. Adds group-node-pending-with-child-done + empty-running-group tests.
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