The clustered write is guarded by to_json's #479 shrink check, but the
`--no-cluster` raw-dump path writes graph.json directly and had no guard, so an
incomplete `--no-cluster` build could still overwrite a larger complete graph
with a partial one — the residual gap noted in the original change.
Add `existing_graph_node_count` in graphify.export (mirrors to_json's guard and
respects the graph-size cap) and, on the raw path, refuse the write with exit 1
before the manifest when the build was incomplete and the new graph has fewer
nodes than the existing one — unless --allow-partial is passed. Both write paths
now enforce the same guarantee.
