Add ping rate tests, discover_peer helper, standardize prepend_buffer to 4096

firmware/lib/arp.cpp

@@ -23,7 +23,7 @@ void handle(std::span<const uint8_t> frame, span_writer& tx,
     if (arp_hdr->oper != ARP_OP_REQUEST) return;
     if (arp_hdr->tpa != our_ip) return;
 
-    prepend_buffer<128> buf;
+    prepend_buffer<4096> buf;
     auto* reply = buf.template prepend<header>();
     reply->htype = ARP_HTYPE_ETH;
     reply->ptype = ARP_PTYPE_IPV4;