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/ashandler.go

@@ -1,6 +1,7 @@
 package routewatch
 
 import (
+	"slices"
 	"sync"
 	"time"
 
@@ -98,18 +99,14 @@ func (h *ASHandler) QueueCapacity() int {
 
 // HandleMessage processes a RIS Live BGP message by extracting all ASNs from
 // the AS path and queuing them for batch insertion into the database. The
-// origin ASN (last element in the path) and all transit ASNs are recorded
-// with their associated timestamps. When the batch reaches the configured size
-// threshold, the flush goroutine is woken to write it.
+// origin ASN and all transit ASNs are recorded with their associated
+// timestamps. When the batch reaches the configured size threshold, the flush
+// goroutine is woken to write it.
 func (h *ASHandler) 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()
 
 	h.mu.Lock()
 	defer h.mu.Unlock()
@@ -129,8 +126,8 @@ func (h *ASHandler) HandleMessage(msg *ristypes.RISMessage) {
 		})
 	}
 
-	// Also track all ASNs in the path
-	for _, asn := range msg.Path {
+	// Also track all ASNs in the path, the members of an AS set included
+	for _, asn := range slices.Concat(msg.Path.Sequence, msg.Path.Set) {
 		if asn > 0 {
 			h.batch = append(h.batch, asnOp{
 				number:    asn,