multiport: make a lane a session on the tunnel, not a tunnel

Lanes used to be separate HostInfos, each with its own handshake, its own
half-established states, its own lifetime and its own slot bookkeeping. That
bought nothing: a lane is the same tunnel over a different underlay 5-tuple.

Derive lane sessions instead. Noise leaves us with A.eKey == B.dKey, so both
sides HKDF-expand the same two keys with the same per-lane label and land on a
matched pair without exchanging anything. Lanes now cost no handshake, have no
half-established state, and die exactly when their base tunnel does. Which lane
a packet belongs to rides the low byte of the nebula header's Reserved field,
inside the AEAD's associated data.

Receiving on a lane needs no permission, since the session exists the moment the
base handshake completes. Sending on one needs proof the new 5-tuple works, so a
lane stays down until a probe on it is acked and falls back to the base tunnel
the moment it stops being acked. Probing is demand-driven off the connection
manager's per-tunnel traffic tick: a peer we exchange a trickle with never costs
more than its base tunnel, however many lanes are configured. The ack rides the
base session on purpose, so a broken reverse lane can't fail a working one.

Because the data now rides lane counters, the rehandshake, exhaustion and
swap-primary checks take the max counter across the base session and its lanes;
otherwise the base counter would sit near zero while a lane ran its keys past
the nonce ceiling.

Removes OutboundLaneTimer, EnsureLanes, startLaneHandshake, completeLane,
completeLaneResponder, makeLaneTrafficDecision and the lane fields on HostInfo.
Handshake payload field 3 (the per-lane handshake index) is permanently
reserved; peers advertise a TxLanes count instead.

handshake/payload_test.go

@@ -146,14 +146,14 @@ func TestPayloadLaneDetails(t *testing.T) {
 		data := MarshalPayload(nil, Payload{
 			InitiatorIndex: 12345,
 			Time:           999,
-			InitiatorLanes: &LaneDetails{PortCount: 8, BasePort: 4242, LaneIndex: 3},
+			InitiatorLanes: &LaneDetails{PortCount: 8, BasePort: 4242, TxLanes: 3},
 			ResponderLanes: &LaneDetails{PortCount: 4, BasePort: 5353},
 		})
 
 		got, err := UnmarshalPayload(data)
 		require.NoError(t, err)
 		require.NotNil(t, got.InitiatorLanes)
-		assert.Equal(t, LaneDetails{PortCount: 8, BasePort: 4242, LaneIndex: 3}, *got.InitiatorLanes)
+		assert.Equal(t, LaneDetails{PortCount: 8, BasePort: 4242, TxLanes: 3}, *got.InitiatorLanes)
 		require.NotNil(t, got.ResponderLanes)
 		assert.Equal(t, LaneDetails{PortCount: 4, BasePort: 5353}, *got.ResponderLanes)
 	})
@@ -237,7 +237,7 @@ func TestPayloadLaneDetails(t *testing.T) {
 
 	t.Run("lane subfield varint overflow rejected", func(t *testing.T) {
 		var lane []byte
-		lane = protowire.AppendTag(lane, fieldLaneLaneIndex, protowire.VarintType)
+		lane = protowire.AppendTag(lane, fieldLaneTxLanes, protowire.VarintType)
 		lane = protowire.AppendVarint(lane, math.MaxUint32+1)
 
 		var details []byte
@@ -442,7 +442,7 @@ func FuzzPayload(f *testing.F) {
 	f.Add(MarshalPayload(nil, Payload{
 		InitiatorIndex: 1,
 		Time:           3,
-		InitiatorLanes: &LaneDetails{PortCount: 8, BasePort: 4242, LaneIndex: 2},
+		InitiatorLanes: &LaneDetails{PortCount: 8, BasePort: 4242, TxLanes: 2},
 		ResponderLanes: &LaneDetails{PortCount: 4, BasePort: 5353},
 	}))
 	f.Add([]byte{})