Zero-copy TX: span_writer packer, static buffers, no vector returns

firmware/include/net.h

@@ -3,14 +3,14 @@
 #include <cstdint>
 #include <functional>
 #include <span>
-#include <vector>
+#include "span_writer.h"
 
 struct net_state {
     std::array<uint8_t, 6> mac;
     std::array<uint8_t, 4> ip;
 };
 
-using net_handler = std::function<std::vector<std::vector<uint8_t>>(std::span<const uint8_t> payload)>;
+using net_handler = std::function<size_t(std::span<const uint8_t> payload, span_writer &out)>;
 
 bool net_init();
 const net_state& net_get_state();