Self-registering IP-protocol and UDP-port dispatch; move picomap-specific constants out of generic net/dispatch layers

firmware/include/udp.h

@@ -39,10 +39,7 @@ void prepend(Buf& buf, const eth::mac_addr& dst_mac, const eth::mac_addr& src_ma
 
 void handle(std::span<const uint8_t> frame, span_writer& tx);
 
-namespace client {
-// Defined by the higher layer (dispatch) to receive decoded UDP payloads
-// addressed to PICOMAP_PORT_BE. Resolved at link time.
-void handler(std::span<const uint8_t> payload, const address& from);
-} // namespace client
+using port_handler = void (*)(std::span<const uint8_t> payload, const address& from);
+void register_port(uint16_t port_be, port_handler fn);
 
 } // namespace udp