Adds vv_disk_io_rates() — snapshots /proc/diskstats each poll and computes per-device MB/s. Exposed as disk_io in the monitor API. JS: - Per-disk rows: ↓read ↑write inline next to disk name (green/amber, hidden when idle) - Pool rows: aggregate pool I/O next to pool name - Array header: total array I/O in the subtitle line Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>