`Scheduler::append_unchecked` was a one-line pass-through to
`Graph::insert_unchecked` with a single caller. `insert_job` is already a
method on `Scheduler`, so it can borrow `self.graph` and call the graph
directly — the wrapper bought nothing but a name.
Also fixes `insert_job`'s doc, which still claimed every node goes
through `Scheduler::append`; it goes straight to the graph's unchecked
insert, and the reason belongs in that doc rather than on a wrapper.
9ab950d767