firmware: split stream recv into its own task with 3-buffer ping-pong

handle_client previously ran recv → decode → paint serially on one
FreeRTOS task. The kernel TCP buffer filled during decode+paint
(~6ms), and against the IDF-default 5760-byte window the sender
naturally stop-go-rate-limited to ~consumption. Raising the window
to 65535 (previous experiment) regressed g2g from ~100ms to 17s
growing unbounded — the sender pumped 45+ segments per round into
a kernel buffer the app couldn't drain in time, and there was no
way to skip-oldest on the kernel queue.

This commit decouples recv from decode+paint:

  recv-task:   owns the socket. Reads header + body into one of three
               preallocated PSRAM body buffers. On body complete, swaps
               the just-filled buffer into a 1-deep pending slot and
               picks a free buffer for the next recv. If the slot
               already held a frame (decode is slow), drops oldest in
               place — mirror of the Scrypted-side skip-oldest from
               e5acf93.

  decode-task: waits on a binary semaphore. On signal, claims pending,
               then decodes + paints without holding any shared lock.
               Frees its prior buffer implicitly by overwriting
               s_decode_idx on the next claim.

3 PSRAM body buffers (~3MB of 28MB free) ensure the invariant
{recv_idx, pending_idx, decode_idx} are pairwise distinct without
ever blocking recv. jpeg_decoder.c grew an alloc_input_buffer helper
+ jpeg_decoder_decode now takes an explicit input pointer so the
stream and http_api snapshot paths don't share scratch.

New stats:
- recv_dropped_oldest: per-window count of pending-slot overwrites
- decode_idle_min/avg/max_us: time decode-task spent waiting on signal

Measurement at IDF-default 5760 window, Unifi medium substream:

  before split: recv_avg=32ms recv_max~44ms fps=22-26 (recv blocked
                during 6ms decode+paint; chunk_max capped at 5760)
  after split:  recv_avg=17ms recv_max=18-37ms fps=21-29 steady,
                decode_idle_avg=27-40ms (decode mostly waiting),
                drop_oldest=0, painted at source rate

The bottleneck moved from 'decode+paint serializes recv' to the
wire's own send rate. Bigger windows are now safe (recv-task drains
continuously, can't bury us), but won't add fps until source rate
goes up — that's a separate conversation.

1 files changed, 5 insertions, 1 deletions

diff --git a/main/http_api.c b/main/http_api.c
index ccd795b..3774039 100644
--- a/main/http_api.c
+++ b/main/http_api.c
@@ -112,6 +112,10 @@ static esp_err_t state_get_handler(httpd_req_t *req)
     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, "recv_dropped_oldest", (double)stream.recv_dropped_oldest);
+    cJSON_AddNumberToObject(s, "decode_idle_min_us",  (double)stream.decode_idle_min_us);
+    cJSON_AddNumberToObject(s, "decode_idle_avg_us",  (double)stream.decode_idle_avg_us);
+    cJSON_AddNumberToObject(s, "decode_idle_max_us",  (double)stream.decode_idle_max_us);
     cJSON_AddNumberToObject(s, "last_paint_event_us_low",
                             (double)stream.last_paint_event_us_low);
     cJSON_AddItemToObject(root, "stream", s);
@@ -424,7 +428,7 @@ static esp_err_t frame_post_handler(httpd_req_t *req)
     size_t back_size = 0;
     void *back = display_back_buffer(&back_size);
     uint16_t w = 0, h = 0;
-    esp_err_t dec_err = jpeg_decoder_decode(got, back, back_size, &w, &h);
+    esp_err_t dec_err = jpeg_decoder_decode(in, got, back, back_size, &w, &h);
     int64_t t_decode = esp_timer_get_time();
     if (dec_err != ESP_OK) {
         viewport_state_lock();