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

firmware/include/handlers.h

@@ -1,9 +1,14 @@
 #pragma once
+#include <cstdint>
 #include <optional>
 #include <string_view>
 #include "dispatch.h"
+#include "ipv4.h"
 #include "wire.h"
 
+inline constexpr uint16_t PICOMAP_PORT_BE = __builtin_bswap16(28781);
+inline constexpr ipv4::ip4_addr PICOMAP_DISCOVERY_GROUP = {239, 112, 77, 1};
+
 extern std::string_view firmware_name;
 
 void handlers_init();