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.

1 files changed, 11 insertions, 0 deletions

diff --git a/main/http_api.c b/main/http_api.c
index 238514b..ccd795b 100644
--- a/main/http_api.c
+++ b/main/http_api.c
@@ -101,6 +101,17 @@ static esp_err_t state_get_handler(httpd_req_t *req)
     cJSON_AddNumberToObject(s, "idle_min_us",   (double)stream.idle_min_us);
     cJSON_AddNumberToObject(s, "idle_avg_us",   (double)stream.idle_avg_us);
     cJSON_AddNumberToObject(s, "idle_max_us",   (double)stream.idle_max_us);
+    // Recv-throughput diagnostics. See stream_server.h for semantics.
+    cJSON_AddNumberToObject(s, "queued_min",      (double)stream.queued_min);
+    cJSON_AddNumberToObject(s, "queued_avg",      (double)stream.queued_avg);
+    cJSON_AddNumberToObject(s, "queued_max",      (double)stream.queued_max);
+    cJSON_AddNumberToObject(s, "recv_calls_min",  (double)stream.recv_calls_min);
+    cJSON_AddNumberToObject(s, "recv_calls_avg",  (double)stream.recv_calls_avg);
+    cJSON_AddNumberToObject(s, "recv_calls_max",  (double)stream.recv_calls_max);
+    cJSON_AddNumberToObject(s, "recv_chunk_min",  (double)stream.recv_chunk_min);
+    cJSON_AddNumberToObject(s, "recv_chunk_avg",  (double)stream.recv_chunk_avg);
+    cJSON_AddNumberToObject(s, "recv_chunk_max",  (double)stream.recv_chunk_max);
+    cJSON_AddNumberToObject(s, "so_rcvbuf",       (double)stream.so_rcvbuf);
     cJSON_AddNumberToObject(s, "last_paint_event_us_low",
                             (double)stream.last_paint_event_us_low);
     cJSON_AddItemToObject(root, "stream", s);