fix: correct INIT padding cap using defines instead of hardcoded 19

Formula: s_max = mtu - 3 - SC_TAG_SIZE + UDP_SC_HDR_SIZE - SC_PUBKEY_ENC_SIZE
For mtu=1460: 1460 - 3 - 16 + 30 - 72 = 1399 (was 1390, off by 9)

src/etcp_connections.c

@@ -133,7 +133,7 @@ static void etcp_link_send_init(struct ETCP_LINK* link, uint8_t reset) {
 
     // padding
     int s = rand() % (link->handshake_maxsize - link->handshake_minsize) + link->handshake_minsize;
-    int s_max = (int)(link->mtu - UDP_HDR_SIZE) - (int)offset - 19;  // 19=3hdr+16tag-72unenc
+    int s_max = (int)(link->mtu) - 3 - (int)SC_TAG_SIZE + (int)UDP_SC_HDR_SIZE - (int)SC_PUBKEY_ENC_SIZE;
     if (s > s_max) s = s_max;
     if (s < 0) s = 0;