firmware: recv-throughput instrumentation (FIONREAD pre-body, recv() call/chunk stats, SO_RCVBUF probe)

Per-frame samples aggregated over the existing 30-frame window:

- queued_at_body_start (FIONREAD just before body recv loop): how much
  of the frame the kernel already absorbed during the previous
  decode+paint. Close to jpeg_len → wire delivered the full frame
  while we were busy (we're decode/paint-bound). Much smaller →
  wire is throttled (window or buffer too small to absorb a frame
  in our paint window).
- recv_calls: number of recv() syscalls the body read needed per
  frame. High → small chunks → window-throttled sender.
- recv_chunk min/avg/max: bytes returned per recv() return in the
  window. Avg = window body bytes / total syscalls.
- SO_RCVBUF: one-shot getsockopt at accept, logged and stashed in
  stats. Confirms whether sdkconfig values reached the build —
  TCP_WND_DEFAULT discrepancies are otherwise invisible.

All surfaced in the windowed log and in /state JSON alongside the
existing recv/dec/paint/idle stats. No behavior change yet.

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