Records the two-part change that gets the maximum raw signal off the
sensors so the future model — and current fine-motion detection —
has everything the parent project describes:
D1 NodeInfo exposes phases[56], n_antennas, noise_floor_dbm,
timestamp_us in the WS payload (was amplitude-only).
D2 NodeState stashes latest phases/noise/timestamp/antenna count
so build_node_features can populate the new fields uniformly
without a parallel phase_history buffer.
D3 csi_keepalive_task spawns managed `ping` children per
discovered sensor address; replaces the operator's hand-run
`ping -i 0.05 …` workflow. CLI --csi-keepalive-pps controls
rate (default 25), 0 disables.
D4 Why ICMP not UDP: sensor rejects closed-port UDP before its
CSI callback fires; ICMP is handled in WiFi RX path regardless.
Verified: 55.6 Hz raw CSI per node with no shell ping; both
amplitude[56] and phases[56] populated; noise_floor=-91 dBm.
Two impl commits already on the branch: 4daa2c9b, 8489efe9.
arsen