metrics_runner computes metrics for an existing session straight to disk; a
generation run's in-memory copy of those items had no metrics and would have
wiped them on its next save. Disk wins for an item's metrics when the memory
item has none (merge_manifest_user_changes, flush_batch_with_vae's disk sync,
the remote VAE worker's disk sync). Fresh in-memory metrics are kept.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>