jobq: test the growth invariants where they are enforced

complete_growing had no test at all -- its only reference was the
internal call from claim_next -- so the rule moved into it in the
previous commit was enforced but unproven.

  - a_completing_node_grows_the_work_it_declared: the declared work
    lands under the emitter, and the emitter parks in Finishing rather
    than going terminal. That ordering is the point of growing as part
    of the completion.
  - a_failed_node_grows_nothing: the rule that moved out of the host.

Both were mutation-checked rather than trusted green: with the
Outcome::Failed guard deleted, a_failed_node_grows_nothing fails on the
appended node while its companion still passes, so the test bites and
the drop is specific to failure rather than blanket.

The departed-parent guard beside it stays untested and says so. Nothing
removes a node from the graph yet (eviction stops retaining a DAG; its
nodes linger) and NodeId cannot be fabricated by construction, so a test
would have to fake the precondition it checks. The comment names the
bounded prune as the point at which it becomes testable.

hive-jobq/src/scheduler.rs

@@ -387,12 +387,18 @@ impl<N, R: Clone + Eq + Hash> Scheduler<N, R> {
         outcome: Outcome,
         grown: JobBuilder<N, R>,
     ) -> Result<(), BuildError> {
-        // A node that is no longer in the graph grows nothing. The DAG it
-        // belonged to can be cancelled or evicted while it runs, and the insert
-        // below is *unchecked* — rooting on a departed parent would plant a
-        // dangling `parent` edge rather than being rejected. The host used to
-        // carry this guard itself, as a lookup before a separate append call;
-        // it belongs here, where the graph is and where it cannot be skipped.
+        // A node that is no longer in the graph grows nothing. The insert below
+        // is *unchecked* — rooting on a departed parent would plant a dangling
+        // `parent` edge rather than being rejected. The host used to carry this
+        // guard itself, as a lookup before a separate append call; it belongs
+        // here, where the graph is and where it cannot be skipped.
+        //
+        // ⚠️ Deliberately untested, and untestable today: nothing removes a node
+        // from the graph yet (eviction only stops *retaining* a DAG; its nodes
+        // linger), and `NodeId` cannot be fabricated, so a test would have to
+        // fake the very condition it checks. This guard is defensive against the
+        // bounded prune that does not exist yet — when that lands, it needs a
+        // test, and this comment is the reminder.
         let grew = if grown.is_empty()
             || matches!(outcome, Outcome::Failed(_))
             || self.graph.node(id).is_none()
@@ -704,6 +710,63 @@ mod tests {
         s.resources.available(&res(name))
     }
 
+    /// Children of `id`, by payload, in insertion order.
+    fn children_of(s: &Scheduler<&'static str, String>, id: NodeId) -> Vec<&'static str> {
+        s.graph()
+            .nodes()
+            .filter(|n| n.parent == Some(id))
+            .map(|n| n.payload)
+            .collect()
+    }
+
+    /// A node completing `Done` gets the work it declared while running,
+    /// inserted **under itself** — so the DAG cannot roll terminal with the
+    /// appended work still pending.
+    #[test]
+    fn a_completing_node_grows_the_work_it_declared() {
+        let mut s = scheduler_with_slots(1);
+        let n = s.append("emitter", vec![], None).expect("insert");
+        assert_eq!(s.settle(), vec![n]);
+
+        let grown = s.new_job();
+        grown.node("child-a");
+        grown.node("child-b");
+        s.complete_growing(n, Outcome::Done, grown)
+            .expect("well-formed growth");
+
+        assert_eq!(children_of(&s, n), vec!["child-a", "child-b"]);
+        // The emitter parks in `Finishing` rather than going terminal: its own
+        // appended work is still pending under it. That ordering is the whole
+        // point of growing *as part of* the completion.
+        assert_eq!(s.graph().node(n).unwrap().state, State::Finishing);
+    }
+
+    /// A **failed** node grows nothing, whatever it declared.
+    ///
+    /// The companion to the test above, and the reason this rule lives in the
+    /// crate rather than in a caller: failure cancel-cascades to every pending
+    /// child of the completing node, so anything inserted here would be
+    /// `Skipped` by the very next statement. Enforcing it host-side means every
+    /// host has to remember it; enforcing it here means none can forget.
+    #[test]
+    fn a_failed_node_grows_nothing() {
+        let mut s = scheduler_with_slots(1);
+        let n = s.append("emitter", vec![], None).expect("insert");
+        assert_eq!(s.settle(), vec![n]);
+
+        let grown = s.new_job();
+        grown.node("never-runs");
+        s.complete_growing(n, Outcome::Failed("boom".to_owned()), grown)
+            .expect("growth is dropped, not rejected");
+
+        assert!(
+            children_of(&s, n).is_empty(),
+            "a failed node must not append work, got {:?}",
+            children_of(&s, n)
+        );
+        assert_eq!(s.graph().node(n).unwrap().state, State::Failed);
+    }
+
     #[test]
     fn leaf_owner_goes_done_directly_and_releases() {
         let mut s = scheduler_with_slots(1);