feat(#2500): validate NodeId references on insert and deserialize

Per mara's direction — validate ids as they enter the graph so internal
iteration can trust every id the graph holds; the generational route for
removal comes later. Adds GraphError; insert() now rejects a dangling
Dep::Node / parent id (it is fallible); validate() checks all internal id
references resolve and that next_id is past the largest existing id;
deserialization runs validate() via #[serde(try_from = "GraphData<N>")], so
a loaded graph can never carry a dangling reference. 5 new tests; serde_json
added as a dev-dependency for the round-trip cases.

hive-jobq/Cargo.toml

@@ -9,3 +9,6 @@ workspace = true
 [dependencies]
 serde = { workspace = true }
 thiserror = { workspace = true }
+
+[dev-dependencies]
+serde_json = { workspace = true }