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>