Use per-node compilation path for via grants in BuildPeerMap and MatchersForNode to ensure via-granted nodes appear in peer maps. Fix ViaRoutesForPeer golden test route inference to correctly resolve via grant effects.
Updates #2180
Add golden test data for via exit route steering and fix via exit grant compilation to match Tailscale SaaS behavior. Includes MapResponse golden tests for via grant route steering verification.
Updates #2180