jobq: stamp created_at on every node, beside started_at/finished_at

`Node` carried two of the three lifecycle timestamps; the third lived on
hive-c0re's `NodeKind::Dag` container payload, a core-specific wrapper the
graph knows nothing about. Give it its real home so the container's copy
becomes redundant rather than load-bearing.

Not an `Option` like its neighbours: starting and finishing are events that
may never happen, but a node that exists was created. Modelling it as
optional would encode a state the graph cannot be in.

`hive-jobq-wire::GraphNode` gains the field in the same commit — it already
carries the other two, and without this one the value cannot reach a viewer
when the container's copy is deleted.

hive-jobq-wire/src/lib.rs

@@ -152,6 +152,9 @@ pub struct GraphNode {
     /// What must hold before this node runs. Omitted when empty.
     #[serde(default, skip_serializing_if = "Vec::is_empty")]
     pub deps: Vec<GraphDep>,
+    /// When the node was inserted into the graph. Always present — a node that
+    /// exists was created, so unlike the two below this is not an `Option`.
+    pub created_at: DateTime<Utc>,
     /// When the node entered `Running`. `None` until it starts; a node that
     /// never ran keeps `None`.
     #[serde(default, skip_serializing_if = "Option::is_none")]
@@ -254,6 +257,7 @@ fn wire_node<N: WireNode, R: WireResource>(node: &hive_jobq::Node<N, R>) -> Grap
         parent: node.parent.map(NodeId::get),
         state: node.state,
         deps: node.deps.iter().map(wire_dep).collect(),
+        created_at: node.created_at,
         started_at: node.started_at,
         finished_at: node.finished_at,
         error: node.error.clone(),
@@ -443,6 +447,7 @@ mod tests {
             parent,
             state,
             deps,
+            created_at: chrono::DateTime::<chrono::Utc>::default(),
             started_at: None,
             finished_at: None,
             error: None,

hive-jobq/src/lib.rs

@@ -255,9 +255,9 @@ impl State {
 }
 
 /// Wall-clock UTC now — the source for node lifecycle timestamps
-/// ([`Node::started_at`] / [`Node::finished_at`]). The graph stamps its own
-/// timestamps rather than threading a clock through every call, so a node's
-/// timing is self-contained. Derived from `SystemTime` (the workspace `chrono`
+/// ([`Node::created_at`] / [`Node::started_at`] / [`Node::finished_at`]). The
+/// graph stamps its own timestamps rather than threading a clock through every
+/// call, so a node's timing is self-contained. Derived from `SystemTime` (the workspace `chrono`
 /// carries no `clock` feature, matching `hive_sh4re::wire_time`), truncated to
 /// whole seconds; a pre-epoch or out-of-range clock clamps to the epoch.
 fn now_utc() -> DateTime<Utc> {
@@ -291,6 +291,14 @@ pub struct Node<N, R> {
     pub deps: Vec<Dep<R>>,
     /// Lifecycle state.
     pub state: State,
+    /// UTC instant the node was inserted into the graph. Stamped by the graph.
+    ///
+    /// Not an `Option`, unlike its two siblings below: starting and finishing
+    /// are events that may never happen, but a node that exists was created.
+    /// Together the three are the node's whole lifecycle — a caller asking
+    /// "how long did this sit before it ran" needs this end of the interval,
+    /// and previously had to get it from a wrapper the graph knows nothing about.
+    pub created_at: DateTime<Utc>,
     /// UTC instant the node entered [`State::Running`] (`None` until it starts;
     /// a cancelled node never ran, so it stays `None`). Stamped by the graph.
     pub started_at: Option<DateTime<Utc>>,
@@ -481,6 +489,7 @@ impl<N, R> Graph<N, R> {
             payload,
             deps,
             state: State::Pending,
+            created_at: now_utc(),
             started_at: None,
             finished_at: None,
             error: None,
@@ -934,6 +943,7 @@ mod tests {
                     when: DepWhen::AFTER_OK,
                 }],
                 state: State::Pending,
+                created_at: now_utc(),
                 started_at: None,
                 finished_at: None,
                 error: None,
@@ -954,6 +964,7 @@ mod tests {
                 payload: "x",
                 deps: vec![],
                 state: State::Pending,
+                created_at: now_utc(),
                 started_at: None,
                 finished_at: None,
                 error: None,