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

firmware/include/dispatch.h

@@ -9,6 +9,8 @@
 #include "prepend_buffer.h"
 #include "udp.h"
 
+uint16_t dispatch_listen_port_be();
+
 struct responder {
     uint32_t message_id;
     udp::address reply_to;
@@ -22,7 +24,7 @@ struct responder {
         if (!r) return;
         buf.append(*r);
         udp::prepend(buf, reply_to.mac, ns.mac, ns.ip, reply_to.ip,
-                     PICOMAP_PORT_BE, reply_to.port, *r);
+                     dispatch_listen_port_be(), reply_to.port, *r);
         net_send_raw(buf.span());
     }
 };
@@ -52,7 +54,7 @@ void typed_handler(const responder& resp, std::span<const uint8_t> payload) {
         resp.respond(*result);
 }
 
-void dispatch_init();
+void dispatch_init(uint16_t listen_port_be);
 timer_handle dispatch_schedule_ms(uint32_t ms, void (*fn)());
 bool dispatch_cancel_timer(timer_handle h);
 [[noreturn]] void dispatch_run(std::span<const handler_entry> handlers);