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.go

@@ -56,11 +56,12 @@ type LaneSuccessStat struct {
 
 // Input feeds the route table builder.
 type Input struct {
-	Sessions      []SessionSnapshot
-	FleetAgents   []FleetAgentSnapshot
-	LaneSuccess   []LaneSuccessStat
-	TargetSubnets []string
-	RequestedLane string
+	Sessions              []SessionSnapshot
+	FleetAgents           []FleetAgentSnapshot
+	LaneSuccess           []LaneSuccessStat
+	TargetSubnets         []string
+	RequestedLane         string
+	ErasureLanesEnabled   bool
 }
 
 // RouteEdge is a weighted edge from a seed hop to a target subnet.
@@ -88,7 +89,8 @@ type RouteRecommendation struct {
 	JoinLane       string  `json:"join_lane,omitempty"`
 	ClearanceLevel int     `json:"clearance_level"`
 	Score          float64 `json:"score"`
-	Reason         string  `json:"reason,omitempty"`
+	Reason              string  `json:"reason,omitempty"`
+	ErasureLanesEnabled bool    `json:"erasure_lanes_enabled,omitempty"`
 }
 
 // SpreadRouteHint is attached to signed deploy plans for agent egress routing.
@@ -102,6 +104,8 @@ type SpreadRouteHint struct {
 	JoinLane       string  `json:"join_lane,omitempty"`
 	Score          float64 `json:"score,omitempty"`
 	ClearanceLevel int     `json:"clearance_level,omitempty"`
+	// ErasureLanesEnabled signals parallel Reed–Solomon lane redundancy on deploy plans.
+	ErasureLanesEnabled bool `json:"erasure_lanes_enabled,omitempty"`
 }
 
 // RouteTable holds weighted edges and recommendations.
@@ -142,7 +146,7 @@ func Build(in Input) *RouteTable {
 	targets := normalizeTargets(in)
 	for _, target := range targets {
 		cands := collectCandidates(in, target, fleetByID, laneRates)
-		rec, edges := scoreCandidates(target, in.RequestedLane, cands)
+		rec, edges := scoreCandidates(target, in.RequestedLane, in.ErasureLanesEnabled, cands)
 		if rec.SeedAgentID != "" {
 			rt.Routes = append(rt.Routes, rec)
 			rt.bySubnet[target] = rec
@@ -168,15 +172,16 @@ func ToHint(rec RouteRecommendation) *SpreadRouteHint {
 		return nil
 	}
 	return &SpreadRouteHint{
-		TargetSubnet:   rec.TargetSubnet,
-		SeedAgentID:    rec.SeedAgentID,
-		SeedAgentName:  rec.SeedAgentName,
-		EgressAgentID:  rec.EgressAgentID,
-		EgressHopIndex: rec.EgressHopIndex,
-		SessionID:      rec.SessionID,
-		JoinLane:       rec.JoinLane,
-		Score:          rec.Score,
-		ClearanceLevel: rec.ClearanceLevel,
+		TargetSubnet:        rec.TargetSubnet,
+		SeedAgentID:         rec.SeedAgentID,
+		SeedAgentName:       rec.SeedAgentName,
+		EgressAgentID:       rec.EgressAgentID,
+		EgressHopIndex:      rec.EgressHopIndex,
+		SessionID:           rec.SessionID,
+		JoinLane:            rec.JoinLane,
+		Score:               rec.Score,
+		ClearanceLevel:      rec.ClearanceLevel,
+		ErasureLanesEnabled: rec.ErasureLanesEnabled,
 	}
 }
 
@@ -292,7 +297,7 @@ func collectCandidates(in Input, target string, fleet map[string]FleetAgentSnaps
 	return out
 }
 
-func scoreCandidates(target, requestedLane string, cands []candidate) (RouteRecommendation, []RouteEdge) {
+func scoreCandidates(target, requestedLane string, erasureLanes bool, cands []candidate) (RouteRecommendation, []RouteEdge) {
 	var edges []RouteEdge
 	var best RouteRecommendation
 	var bestScore float64
@@ -331,16 +336,17 @@ func scoreCandidates(target, requestedLane string, cands []candidate) (RouteReco
 				reason = "fleet agent on target subnet"
 			}
 			best = RouteRecommendation{
-				TargetSubnet:   target,
-				SeedAgentID:    c.agentID,
-				SeedAgentName:  c.agentName,
-				EgressAgentID:  c.agentID,
-				EgressHopIndex: c.hopIndex,
-				SessionID:      c.sessionID,
-				JoinLane:       firstNonEmpty(requestedLane, c.joinLane),
-				ClearanceLevel: c.clearance,
-				Score:          weight,
-				Reason:         reason,
+				TargetSubnet:        target,
+				SeedAgentID:         c.agentID,
+				SeedAgentName:       c.agentName,
+				EgressAgentID:       c.agentID,
+				EgressHopIndex:      c.hopIndex,
+				SessionID:           c.sessionID,
+				JoinLane:            firstNonEmpty(requestedLane, c.joinLane),
+				ClearanceLevel:      c.clearance,
+				Score:               weight,
+				Reason:              reason,
+				ErasureLanesEnabled: erasureLanes,
 			}
 		}
 	}

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,