feat: send_q backpressure + ll_queue on_get callback + consumer_ack flow control

etcp_router: ROUTER_MAX_SEND_Q_PACKETS=256, router_enqueue_send returns
error when send_q full, etcp_route_send propagates -1 on backpressure.

ll_queue: queue_set_on_get — deferred callback after each queue_data_get.
Replaces manual consumer_ack polling with queue-driven notification.

tcp_proxy_server: consumer_ack=1, write_on_get_cb triggers ACK on
each write_queue dequeue. Full backpressure chain:
  write_queue full → no dequeue → no ACK → sender stops → all queues pause
  write_queue drains → dequeue → on_get → ACK → sender resumes

lib/ll_queue.c

@@ -47,6 +47,7 @@ static inline void queue_check_thread(struct ll_queue* q) {
 static void queue_resume_timeout_cb(void* arg);
 static void check_waiters(struct ll_queue* q);
 static void empty_trampoline(void* arg);
+static void on_get_trampoline(void* arg);
 
 static void empty_trampoline(void* arg) {
     struct ll_queue* q = (struct ll_queue*)arg;
@@ -58,6 +59,11 @@ static void empty_trampoline(void* arg) {
     if (q->count == 0 && cb) cb(q, cb_arg);
 }
 
+static void on_get_trampoline(void* arg) {
+    struct ll_queue* q = (struct ll_queue*)arg;
+    if (q->on_get) q->on_get(q, q->on_get_arg);
+}
+
 // ==================== Управление очередью ====================
 
 struct ll_queue* queue_new(struct UASYNC* ua, size_t hash_size, uint16_t index_offset, uint16_t index_size, char* name) {
@@ -151,6 +157,8 @@ void queue_free(struct ll_queue* q) {
     if (old_eid) uasync_call_soon_cancel(q->ua, old_eid);
     q->empty_callback = NULL;
     q->empty_callback_arg = NULL;
+    q->on_get = NULL;
+    q->on_get_arg = NULL;
     
     u_free(q);
 }
@@ -552,6 +560,9 @@ struct ll_entry* queue_data_get(struct ll_queue* q) {
     if (q->count == 0 && q->empty_callback && !q->empty_call_soon_id)
         q->empty_call_soon_id = uasync_call_soon(q->ua, q, (timeout_callback_t)empty_trampoline);
     
+    if (q->on_get)
+        uasync_call_soon(q->ua, q, (timeout_callback_t)on_get_trampoline);
+    
 #ifdef QUEUE_DEBUG
     queue_check_consistency(q);// !!!! for debug
 #endif
@@ -644,6 +655,12 @@ void queue_set_empty_callback(struct ll_queue* q, queue_callback_fn cbk_fn, void
         q->empty_call_soon_id = uasync_call_soon(q->ua, q, (timeout_callback_t)empty_trampoline);
 }
 
+void queue_set_on_get(struct ll_queue* q, queue_callback_fn cbk_fn, void* arg) {
+    if (!q) return;
+    q->on_get = cbk_fn;
+    q->on_get_arg = arg;
+}
+
 int queue_waiter_wait(struct ll_queue* q, struct queue_waiter_handle* h,
                       queue_threshold_callback_fn callback, void* arg) {
     if (!q || !h) return -1;