test(context-loader): cover a seed that is an edge source but not a node

The differential suite compared declared nodes and ids absent from the graph
entirely, which conflates two different things: an id with no node and no edges,
and an id with no node but with outgoing edges. Only the second makes it
observable whether the traversal short-circuits on an undeclared seed.

It matters because short-circuiting there looks like an obvious improvement —
the documented behaviour for an unknown seed is an empty result — but the
framework's traversal follows the edges instead, so the two would silently stop
agreeing. Without this case the whole suite stays green while that happens.

core/wren-context-loader/tests/impact_differential.rs

@@ -268,6 +268,19 @@ fn a_cyclic_graph_terminates_and_still_agrees() {
     );
 }
 
+/// A seed that is an edge *source* but not a declared node. Distinct from a dangling endpoint:
+/// here the undeclared id has outgoing edges, so "unknown seed" and "leaf seed" come apart, and
+/// whether the traversal short-circuits on an undeclared seed becomes observable.
+#[test]
+fn a_dangling_edge_source_agrees_too() {
+    let graph = LineageGraph {
+        nodes: vec![node("dashboard:sales", LineageKind::Dashboard)],
+        edges: vec![edge("model:missing", "dashboard:sales")],
+    };
+    assert_agrees_on(&graph, "model:missing");
+    assert_agrees_everywhere(&graph);
+}
+
 #[test]
 fn a_dangling_edge_endpoint_carries_no_severity() {
     // `is_resolvable` is what reports a dangling reference; the impact analysis must not invent a