The CSI callback fires for every WiFi frame in promiscuous mode
(100-500+ fps). Each call invoked sendto() synchronously, exhausting
lwIP packet buffers (errno 12 = ENOMEM). The rapid-fire failures
cascaded into a LoadProhibited guru meditation crash.
Two fixes:
1. csi_collector.c: Rate-limit UDP sends to 50 Hz (20ms interval).
CSI frames arriving between sends are dropped — the sensing
pipeline only needs 20-50 Hz.
2. stream_sender.c: When sendto fails with ENOMEM, suppress further
sends for 100ms to let lwIP reclaim buffers. Logs the backoff
event and resumes automatically.
Closes#127
ce171696b2