Rework the crate's scheduling model onto an explicit parent (grouping)
axis, separate from the dep (ordering) axis.
- Node gains a structural `parent: Option<NodeId>`, set by the caller
independent of its `Dep::Node` edges. Grouping is not ordering. A
`Dep::Node` edge must stay inside the depender's own parent group
(validated) — never crossing to another group or onto the parent.
- Resource holding walks the parent tree: acquire fresh when no ancestor
holds it (the acquirer owns it, held for its whole subtree); borrow an
ancestor's grant (one branch at a time; nodes inside are covered); take
an extra unit when the grant is lent to a sibling branch, else wait. A
grant releases only once the owner and its whole subtree are terminal.
- Completion rolls up the parent tree: a node's sub-nodes run after its
own logic, and it is not terminal until they finish — it parks in
`State::Finishing`, rolling up to Done (every child Done) or Failed
(any child Failed/Cancelled). A child is gated on its parent reaching
Finishing; a downstream dep on a node therefore waits for that node's
dynamically-appended children with no explicit edge. A failed node
cancels its pending sub-nodes.
Deletes the SharedResources/ResourceGuard layer (guard.rs) and the
add_dep graph-growth hook (no longer needed). The scheduler stays
single-threaded, owning the ResourceTable directly. Early release of a
grant once no subtree node still needs it is a deferred optimization
(unsafe under dynamically-appended subnodes, #2611).
Base for the hive-c0re job_queue port (#2605), split out so that PR can
rebase onto it.
5906cc2f2b