jobq: make NodeGuid an actual guid

Third time the operator asked for a guid and got a substitute: first an
i64 index, then a {random job id, per-builder counter} pair. The pair was
defensible in isolation -- a foreign handle misses rather than colliding,
with no new dependency -- but "an equivalent that avoids a dep" is a
counter-proposal, not an implementation.

It is also simpler as a guid, which was the question asked: NodeGuid(Uuid)
drops the `job` field, the `next_seq` counter, `fresh_job_id()` and the
`Cell` import, and halves the type's doc. One random draw per node rather
than one per builder -- noise next to what a node does when it runs.

uuid 1.24 was already in Cargo.lock as a transitive dependency, so this
adds an edge rather than a package.

Cargo.lock

@@ -1758,6 +1758,7 @@ dependencies = [
  "serde",
  "serde_json",
  "thiserror 2.0.18",
+ "uuid",
 ]
 
 [[package]]