policy/v2,state,mapper: implement per-viewer via route steering

Via grants steer routes to specific nodes per viewer. Until now,
all clients saw the same routes for each peer because route
assembly was viewer-independent. This implements per-viewer route
visibility so that via-designated peers serve routes only to
matching viewers, while non-designated peers have those routes
withdrawn.

Add ViaRouteResult type (Include/Exclude prefix lists) and
ViaRoutesForPeer to the PolicyManager interface. The v2
implementation iterates via grants, resolves sources against the
viewer, matches destinations against the peer's advertised routes
(both subnet and exit), and categorizes prefixes by whether the
peer has the via tag.

Add RoutesForPeer to State which composes global primary election,
via Include/Exclude filtering, exit routes, and ACL reduction.
When no via grants exist, it falls back to existing behavior.

Update the mapper to call RoutesForPeer per-peer instead of using
a single route function for all peers. The route function now
returns all routes (subnet + exit), and TailNode filters exit
routes out of the PrimaryRoutes field for HA tracking.

Updates #2180

hscontrol/state/state.go

@@ -1061,6 +1061,44 @@ func (s *State) GetNodePrimaryRoutes(nodeID types.NodeID) []netip.Prefix {
 	return s.primaryRoutes.PrimaryRoutes(nodeID)
 }
 
+// RoutesForPeer computes the routes a peer should advertise to a specific viewer,
+// applying via grant steering on top of global primary election and exit routes.
+// When no via grants apply, this falls back to existing behavior (global primaries + exit routes).
+func (s *State) RoutesForPeer(
+	viewer, peer types.NodeView,
+	matchers []matcher.Match,
+) []netip.Prefix {
+	viaResult := s.polMan.ViaRoutesForPeer(viewer, peer)
+
+	globalPrimaries := s.primaryRoutes.PrimaryRoutes(peer.ID())
+	exitRoutes := peer.ExitRoutes()
+
+	// Fast path: no via grants affect this pair — existing behavior.
+	if len(viaResult.Include) == 0 && len(viaResult.Exclude) == 0 {
+		allRoutes := slices.Concat(globalPrimaries, exitRoutes)
+
+		return policy.ReduceRoutes(viewer, allRoutes, matchers)
+	}
+
+	// Remove excluded routes (steered to a different peer for this viewer).
+	var routes []netip.Prefix
+
+	for _, p := range slices.Concat(globalPrimaries, exitRoutes) {
+		if !slices.Contains(viaResult.Exclude, p) {
+			routes = append(routes, p)
+		}
+	}
+
+	// Add included routes (this peer is via-designated for this viewer).
+	for _, p := range viaResult.Include {
+		if !slices.Contains(routes, p) {
+			routes = append(routes, p)
+		}
+	}
+
+	return policy.ReduceRoutes(viewer, routes, matchers)
+}
+
 // PrimaryRoutesString returns a string representation of all primary routes.
 func (s *State) PrimaryRoutesString() string {
 	return s.primaryRoutes.String()