stream: instrument TCP-window decomposition (wire kbps, hdr_gap, pend_age)

Before touching CONFIG_LWIP_TCP_WND_DEFAULT, make the window question
decidable from the logs. New per-window metrics in the stream log,
/state, and stats struct:

- wire min/avg/max kbps: instantaneous throughput while each body
  drained (jpeg_len/recv_us). Ceiling ~= TCP_WND/RTT, so it scales
  with the window iff the window is the limiter.
- hdr_gap min/avg/max us: time blocked waiting for the next header
  after finishing a body. Large = sender-paced; ~0 = receive path
  is the bottleneck.
- pend_age min/avg/max us: publish->claim latency of painted frames.
  Growing across windows = queue backlog building, the failure mode
  that killed the previous WND=65535 attempt.

Together with recv/dec/paint the frame interval is now fully
decomposable: interval ~= hdr_gap + recv + pend_age + dec + paint.

Also stamp TCP_WND/TCP_MSS/RECVMBOX into the client-connect log line
so every capture is self-labeled with the config it ran under.

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