types/netmap,wgengine/magicsock: propagate CapVer to magicsock.endpoint (#16244)

This enables us to mark nodes as relay capable or not. We don't actually
do that yet, as we haven't established a relay CapVer.

Updates tailscale/corp#27502

Signed-off-by: Jordan Whited <jordan@tailscale.com>

types/netmap/nodemut.go

@@ -69,6 +69,17 @@ func (m NodeMutationLastSeen) Apply(n *tailcfg.Node) {
 	n.LastSeen = ptr.To(m.LastSeen)
 }
 
+// NodeMutationCap is a NodeMutation that says a node's
+// [tailcfg.CapabilityVersion] value has changed.
+type NodeMutationCap struct {
+	mutatingNodeID
+	Cap tailcfg.CapabilityVersion
+}
+
+func (m NodeMutationCap) Apply(n *tailcfg.Node) {
+	n.Cap = m.Cap
+}
+
 var peerChangeFields = sync.OnceValue(func() []reflect.StructField {
 	var fields []reflect.StructField
 	rt := reflect.TypeFor[tailcfg.PeerChange]()
@@ -105,6 +116,8 @@ func NodeMutationsFromPatch(p *tailcfg.PeerChange) (_ []NodeMutation, ok bool) {
 			ret = append(ret, NodeMutationOnline{mutatingNodeID(p.NodeID), *p.Online})
 		case "LastSeen":
 			ret = append(ret, NodeMutationLastSeen{mutatingNodeID(p.NodeID), *p.LastSeen})
+		case "Cap":
+			ret = append(ret, NodeMutationCap{mutatingNodeID(p.NodeID), p.Cap})
 		}
 	}
 	return ret, true