add p2p-relay transport router scaffold for nodes tool

pkg/nodes/transport.go

@@ -0,0 +1,66 @@
+package nodes
+
+import (
+	"context"
+	"fmt"
+	"strings"
+)
+
+// Transport abstracts node data-plane delivery.
+type Transport interface {
+	Name() string
+	Send(ctx context.Context, req Request) (Response, error)
+}
+
+// Router prefers p2p transport and falls back to relay.
+type Router struct {
+	P2P   Transport
+	Relay Transport
+}
+
+func (r *Router) Dispatch(ctx context.Context, req Request, mode string) (Response, error) {
+	m := strings.ToLower(strings.TrimSpace(mode))
+	if m == "" {
+		m = "auto"
+	}
+	switch m {
+	case "p2p":
+		if r.P2P == nil {
+			return Response{OK: false, Node: req.Node, Action: req.Action, Error: "p2p transport unavailable"}, nil
+		}
+		return r.P2P.Send(ctx, req)
+	case "relay":
+		if r.Relay == nil {
+			return Response{OK: false, Node: req.Node, Action: req.Action, Error: "relay transport unavailable"}, nil
+		}
+		return r.Relay.Send(ctx, req)
+	default: // auto
+		if r.P2P != nil {
+			if resp, err := r.P2P.Send(ctx, req); err == nil && resp.OK {
+				return resp, nil
+			}
+		}
+		if r.Relay != nil {
+			return r.Relay.Send(ctx, req)
+		}
+		return Response{}, fmt.Errorf("no transport available")
+	}
+}
+
+// StubP2PTransport provides phase-2 negotiation scaffold.
+type StubP2PTransport struct{}
+
+func (s *StubP2PTransport) Name() string { return "p2p" }
+func (s *StubP2PTransport) Send(ctx context.Context, req Request) (Response, error) {
+	_ = ctx
+	return Response{OK: false, Node: req.Node, Action: req.Action, Error: "p2p session not established yet"}, nil
+}
+
+// StubRelayTransport provides executable placeholder until real bridge lands.
+type StubRelayTransport struct{}
+
+func (s *StubRelayTransport) Name() string { return "relay" }
+func (s *StubRelayTransport) Send(ctx context.Context, req Request) (Response, error) {
+	_ = ctx
+	return Response{OK: false, Node: req.Node, Action: req.Action, Error: "relay bridge not implemented yet"}, nil
+}