Fix relay re-establishment for handshake on Disestablised entry (#1805)

handleOutsideRelayPacket filled ViaSender.remoteIdx with relay.RemoteIndex,
an index from the relay peer's index space, but the rescue in
sendHandshakeResponse looks that value up in relayForByIdx, which is keyed
by local index. The lookup could never hit, so a terminal relay entry left
Disestablished by a one-sided teardown stayed Disestablished even after a
valid handshake arrived over it. The responder's first transmit then failed
to find an Established relay, deleted its only relay entry, and every
subsequent send was silently dropped until dead-tunnel detection forced a
re-handshake.

hostmap.go

@@ -287,7 +287,6 @@ type HostInfo struct {
 type ViaSender struct {
 	UdpAddr   netip.AddrPort
 	relayHI   *HostInfo // relayHI is the host info object of the relay
-	remoteIdx uint32    // remoteIdx is the index included in the header of the received packet
 	relay     *Relay    // relay contains the rest of the relay information, including the PeerIP of the host trying to communicate with us.
 	IsRelayed bool      // IsRelayed is true if the packet was sent through a relay
 }