docs: mark six deferred review findings with TODOs

Behavior untouched; each marker records a known gap and the intended
fix so the next visit doesn't rediscover it: transient zero-sent
sendmmsg errors drop a whole run; the non-vnet Poll queue lacks the
post-wake drain loop; recvmmsg controllen resets touch every entry;
cached handshake packets flush one syscall each; the routines clamp in
activate() would blackhole surplus REUSEPORT sockets if it ever became
reachable; darwin WriteBatch burst-drops on EWOULDBLOCK.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014ugV2edVqoz3tBvq9J6yWp

handshake_manager.go

@@ -975,7 +975,9 @@ func (hm *HandshakeManager) continueHandshake(via ViaSender, hh *HandshakeHostIn
 		nb := make([]byte, 12, 12)
 		out := make([]byte, mtu)
 		for _, cp := range hh.packetStore {
-			//todo use a sendbatcher
+			// TODO: use a SendBatch here. Each callback lands in
+			// sendNoMetrics -> WriteTo: one syscall per cached packet,
+			// where one sendmmsg could flush the whole store.
 			cp.callback(cp.messageType, cp.messageSubType, hostinfo, cp.packet, nb, out)
 		}
 		f.cachedPacketMetrics.sent.Inc(int64(len(hh.packetStore)))