refactor(#2949): the mutex holds the scheduler, not a wrapper

`QueueInner` existed to hold the scheduler *and* a per-node side map. The
map is gone, so it was a struct around one field — and worse, a struct of
a type `hive_jobq` cannot drive: the crate's run-loop seam takes
`&Arc<Mutex<Scheduler<..>>>` specifically.

So `JobQueue` now holds `Arc<Mutex<Sched>>` directly, where `Sched` is
just `Scheduler<NodeKind, Resource>`. Its six methods become free
functions over `&Sched`; all six are `DagView` projections, i.e. the code
the endpoint rework is going to delete anyway, so this does not entrench
them.

This is the precondition for c0re calling `claim_next`, not that switch
itself — `run_worker` still claims through `claim_ready`. Landing it
separately keeps the type change reviewable on its own.

hive-c0re/src/job_queue/tests.rs

@@ -117,7 +117,6 @@ fn settle_rebuild_tail(q: &JobQueue, agent: &str, expect_ok: bool) {
 fn declared_resources(q: &JobQueue, node_id: hive_jobq::NodeId) -> Vec<Resource> {
     let inner = q.lock();
     inner
-        .sched
         .graph()
         .node(node_id)
         .expect("node exists")