Keep a trailing AS set out of the origin and the peerings (closes #75)

A path ending in an AS set, such as [64496, 64497, [64498, 64499]], was
flattened into the sequence, so the route was stored as originated by the
last set member and the peering handler recorded peerings the path does not
show. ASPath now keeps the sequence and a trailing set of several members
apart; Origin() gives the last AS of the sequence, or 0 (no known origin)
for such a set. The handlers use it; peerings come from the sequence only.
The pages show origin 0 as no known origin, and the IP lookup asks no WHOIS
lookup for it. Empty paths are unchanged.

Model: opus-5-5

internal/routewatch/prefixhandler.go

@@ -73,6 +73,9 @@ type prefixUpdate struct {
 	messageType string
 	timestamp   time.Time
 	path        []int
+	// endsInSet says the path ends in an AS set of several members, so it
+	// gives no origin.
+	endsInSet bool
 }
 
 // NewPrefixHandler creates a new batched prefix tracking handler
@@ -127,11 +130,11 @@ func (h *PrefixHandler) HandleMessage(msg *ristypes.RISMessage) {
 	// Use the pre-parsed timestamp
 	timestamp := msg.ParsedTimestamp
 
-	// Get origin ASN from path (last element)
-	var originASN int
-	if len(msg.Path) > 0 {
-		originASN = msg.Path[len(msg.Path)-1]
-	}
+	originASN := msg.Path.Origin()
+
+	// The stored and shown AS path lists the members of an AS set that ends
+	// the path after the rest of it.
+	path := slices.Concat(msg.Path.Sequence, msg.Path.Set)
 
 	h.mu.Lock()
 	defer h.mu.Unlock()
@@ -169,7 +172,8 @@ func (h *PrefixHandler) HandleMessage(msg *ristypes.RISMessage) {
 				peer:        msg.Peer,
 				messageType: "announcement",
 				timestamp:   timestamp,
-				path:        msg.Path,
+				path:        path,
+				endsInSet:   len(msg.Path.Set) > 0,
 			})
 			// Record announcement in metrics
 			if h.metrics != nil {
@@ -297,7 +301,11 @@ func (h *PrefixHandler) flushBatch() {
 			prefixesToUpdate[update.prefix] = update.timestamp
 		}
 
-		if update.messageType == "announcement" && update.originASN > 0 {
+		// An announcement is stored when its path gives an origin, or when it
+		// ends in an AS set of several members: that route is stored with
+		// origin 0 (none). An announcement with an empty path is not stored.
+		if update.messageType == "announcement" &&
+			(update.originASN > 0 || update.endsInSet) {
 			// Create live route for batch upsert
 			route := h.createLiveRoute(update)
 			if route != nil {