refactor(#2908): resolve a wire id in the graph, not the c0re layer

mara on !2909: "shouldnt node_by_id be part of jobq?" — yes. Resolving a
raw value to a `NodeId` is the exact inverse of `NodeId::get`, which
already lives in hive-jobq, and it is only a search because the graph
owns the counter that makes ids unfabricable. Both halves of that
round-trip belong on the same side of the crate boundary.

Placing it in `QueueInner` also put it in a layer slated for removal, so
the c0re-side helper would have had to move later anyway — and it was
private there, leaving any other caller needing the same resolution to
write the same `nodes().find_map(…)` by hand.

`Graph::resolve_id` replaces it, with a unit test covering the
round-trip and the rejection of a value that was never an id.

While re-reading the diff for that question: the doc comment added in
the previous commit landed *between* `container`'s doc comment and its
signature, silently reattaching "The container node of `dag_id`" to the
new helper and leaving `container` undocumented. Restored.

No behaviour change and no wire change — same search, same call site.
Checked with clippy (`--all-targets -D warnings`), `cargo test -p
hive-jobq -p hive-c0re` (41 + 322 passed) and `nix fmt`.

hive-jobq/src/lib.rs

@@ -461,6 +461,21 @@ impl<N, R> Graph<N, R> {
         self.nodes.iter().find(|n| n.id == id)
     }
 
+    /// Resolve a raw value back to the opaque [`NodeId`] it names — the inverse
+    /// of [`NodeId::get`], and the only way to perform that direction. A caller
+    /// holding a value that crossed a wire cannot fabricate an id from it (that
+    /// impossibility is the point of the type), so it has to be matched against
+    /// the graph, which is what makes this a search rather than a cast.
+    ///
+    /// `None` when no node carries that value, which covers both a value that
+    /// was never an id and one whose node has since been reaped.
+    #[must_use]
+    pub fn resolve_id(&self, raw: u64) -> Option<NodeId> {
+        self.nodes
+            .iter()
+            .find_map(|n| (n.id.0 == raw).then_some(n.id))
+    }
+
     /// Every node in the graph, in insertion order. The scheduler iterates
     /// this to find runnable pending nodes.
     pub fn nodes(&self) -> impl Iterator<Item = &Node<N, R>> {
@@ -729,6 +744,20 @@ mod tests {
         );
     }
 
+    #[test]
+    fn resolve_id_inverts_get_and_rejects_a_value_that_was_never_an_id() {
+        let mut g: Graph<&str, String> = Graph::new();
+        let a = g.insert("a", vec![], None).unwrap();
+        let b = g.insert("b", vec![], None).unwrap();
+        // Round-trips every id the graph handed out: this is the only way back
+        // from a raw value, since NodeId can't be constructed from one.
+        assert_eq!(g.resolve_id(a.get()), Some(a));
+        assert_eq!(g.resolve_id(b.get()), Some(b));
+        // A value that was never an id resolves to nothing, so a caller can't
+        // reach a node by guessing a number off the wire.
+        assert_eq!(g.resolve_id(u64::MAX), None);
+    }
+
     #[test]
     fn state_terminality() {
         assert!(State::Done.is_terminal());