Opt-in per-command and per-sample dump to configurable dump_dir.
Config-gated via [debug] dump_dir, no change when unset. Serialized
through a single writer GenServer to avoid interleaving.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Compact per-pool block with type, capacity bar, used/free/total,
scrub state, and vdev summary. Collector gets pool_type derivation,
scan state, and vdev list — no new shell-outs.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
