The raw-graph wire drops the inline build_log_id; the client fetches a
node's captured output on demand. Two handlers resolve node id -> log-row
id (JobQueue::build_log_id_of, now keyed by the wire u64) then delegate to
the existing get_full / raw handlers: /api/build-log/<node_id> serves the
BuildLogFull JSON ({stdout, stderr} + header), /raw serves text/plain.
404 when the node has no linked log.
