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.

1 files changed, 16 insertions, 0 deletions

diff --git a/main/stream_server.h b/main/stream_server.h
index e2eba06..2562738 100644
--- a/main/stream_server.h
+++ b/main/stream_server.h
@@ -73,6 +73,22 @@ typedef struct {
     uint32_t last_paint_event_us_low;   // last v1 frame's event_us_low,
                                         // 0 if none seen yet on this
                                         // boot or last frame was v0
+    // TCP-window decomposition. Together with recv/dec/paint these account
+    // for the whole frame interval:
+    //   interval ≈ hdr_gap (sender idle) + recv (wire) + pend_age (handoff
+    //   queue wait) + dec + paint.
+    // wire = instantaneous throughput while the body drained (jpeg_len /
+    // recv_us). Ceiling ≈ TCP_WND / RTT — the definitive window-throttle
+    // metric: scales with CONFIG_LWIP_TCP_WND iff the window is the limiter.
+    uint32_t wire_min_kbps, wire_avg_kbps, wire_max_kbps;
+    // Blocked-with-nothing-to-read time between previous body completion
+    // and next header completion. Large → sender-paced (we are NOT the
+    // bottleneck); ~0 → back-to-back arrivals, receive path is the limiter.
+    uint32_t hdr_gap_min_us, hdr_gap_avg_us, hdr_gap_max_us;
+    // publish → decode-claim latency of painted frames. Healthy: bounded
+    // well under a frame interval. Growing window-over-window = latency
+    // backlog building (the failure mode that killed the last WND raise).
+    uint32_t pend_age_min_us, pend_age_avg_us, pend_age_max_us;
 } stream_server_stats_t;
 
 void stream_server_snapshot_stats(stream_server_stats_t *out);