Zero-copy encode: pack response body in place, single shared tx_buf, drain rx loop

firmware/include/net.h

@@ -15,4 +15,4 @@ using net_handler = std::function<size_t(std::span<const uint8_t> payload, span_
 bool net_init();
 const net_state& net_get_state();
 void net_set_handler(net_handler handler);
-void net_poll();
+void net_poll(std::span<uint8_t> tx);