Add erasure-coded multi-lane spread foundation.

Server Reed-Solomon 4+2 shard encode on deploy plans when erasure_lanes_enabled; agents reassemble from parallel lane URLs as staging fallback with BGP/Path Tracer hints.

server/internal/spreadrouter/router_test.go

@@ -104,6 +104,25 @@ func TestNormalizeSubnet(t *testing.T) {
 	}
 }
 
+func TestBuildSetsErasureLanesFlag(t *testing.T) {
+	in := Input{
+		TargetSubnets:         []string{"10.9.8"},
+		ErasureLanesEnabled:   true,
+		FleetAgents: []FleetAgentSnapshot{
+			{AgentID: "a1", Subnet: "10.9.8", Clearance: clearance.L2, Connected: true},
+		},
+	}
+	rt := Build(in)
+	rec, ok := rt.Recommend("10.9.8")
+	if !ok || !rec.ErasureLanesEnabled {
+		t.Fatalf("route=%+v ok=%v", rec, ok)
+	}
+	hint := ToHint(rec)
+	if hint == nil || !hint.ErasureLanesEnabled {
+		t.Fatalf("hint=%+v", hint)
+	}
+}
+
 func TestToHint(t *testing.T) {
 	hint := ToHint(RouteRecommendation{
 		TargetSubnet: "10.1.2", SeedAgentID: "a1", EgressAgentID: "a1", Score: 0.8,