fix: clean up misc TODO comments and improve UDP source filtering (#89)

- http/httpraw: cap header slice growth to pre-allocated capacity, fix
  benchmark to reuse Header across iterations (0 allocs/op)
- internal/ring: replace TODO panic comments with invariant explanations
- tcp/conn: document truncated-frame offset check
- tcp/handler: replace TODO with net.ErrClosed on RST-closed connection
- internet/stack-udpport: filter incoming packets by remote IP address
  when configured via SetStackNode; skip filter for IPv4 multicast
  destinations (class D) so mDNS and similar protocols work correctly

Generated with LLM assistance.

Signed-off-by: Marvin Drees <marvin.drees@9elements.com>

x/xnet/xnet_mdns_test.go

@@ -297,7 +297,9 @@ func newMDNSStack(t *testing.T, hostname string, seed int64,
 		t.Fatal(hostname, "mdns configure:", err)
 	}
 
-	err = stack.RegisterUDP4(&client, addr.As4(), mdns.Port)
+	var remote [4]byte
+	copy(remote[:], mdnsCfg.MulticastAddr)
+	err = stack.RegisterUDP4(&client, remote, mdns.Port)
 	if err != nil {
 		t.Fatal(hostname, "register udp:", err)
 	}