jobq: complete is the completion, and failure grows nothing

Two review findings from the previous round, re-checked against the
actual tree rather than against my notes.

`Scheduler::complete` was still a public wrapper whose entire body was
`self.finish(id, outcome)`. Its docstring argued the split was not a
redirect because both completion forms shared `finish` -- but sharing a
private helper is not a reason for two public names. `finish`'s body now
lives in `complete`, and `complete_growing` calls it. Same sharing, one
name, no redirect.

Growth on a failed node is now dropped by `complete_growing` instead of
by the host loop. Failure cancel-cascades to every pending child of the
completing node, and grown work is inserted as its children, so anything
appended here is Skipped by the next statement -- the insert is not
wrong, it is provably pointless. That is a consequence of this crate's
cascade rule, so this crate should be the one enforcing it; a host that
has to remember it can forget it. Behaviour is unchanged: hive-c0re
already dropped growth before calling, and now no longer has to.

`Scheduler::new_job` is left alone but documented for what it is: the
hole in `JobBuilder::new`'s pub(crate) wall, with no non-test caller
since claim_next mints a builder per running node. Closing it is a venue
question rather than a rename, so it stays for now.

hive-c0re/src/job_queue/mod.rs

@@ -252,22 +252,6 @@ impl JobQueue {
         self.lock().graph().root_of(node).map(NodeId::get)
     }
 
-    /// A builder for a node to declare more work into while it runs.
-    ///
-    /// Handed to [`exec::run_node`] and returned to the crate's completion.
-    /// Only `hive_jobq` can construct one, which is why this goes through the
-    /// scheduler rather than `Job::default()`.
-    ///
-    /// The completion wrappers that used to live beside this (`complete_node`,
-    /// `complete_node_growing`) are **gone**: a node is completed inside the
-    /// future [`hive_jobq::scheduler::Scheduler::claim_next`] hands back, so
-    /// this layer has nothing left to wrap. The tests keep their own extension
-    /// trait for driving completions by hand.
-    #[must_use]
-    pub fn new_job(&self) -> Job {
-        self.lock().new_job()
-    }
-
     /// Cancel a DAG that hasn't started yet: every work node is still `Pending`,
     /// so each is cancelled. `false` once any work node is running or terminal —
     /// an in-flight nix build isn't interruptible.