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/api/erasure_auth_test.go

@@ -0,0 +1,38 @@
+package api
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestAuthSpreadPolicyIncludesErasureLanes(t *testing.T) {
+	hub := NewWSHub(nil)
+	hub.SetServerPolicy(ServerPolicy{ErasureLanesEnabled: true})
+	raw := buildAuthSpreadPolicy(hub)
+	var policy map[string]interface{}
+	if err := json.Unmarshal(raw, &policy); err != nil {
+		t.Fatal(err)
+	}
+	enabled, ok := policy["erasure_lanes_enabled"].(bool)
+	if !ok || !enabled {
+		t.Fatalf("policy=%#v", policy)
+	}
+}
+
+func buildAuthSpreadPolicy(hub *WSHub) json.RawMessage {
+	policy := hub.serverPolicySnapshot()
+	if !policy.ErasureLanesEnabled && policy.HashrateGateSpreadMin <= 0 && policy.HashrateGateHPS <= 0 {
+		return nil
+	}
+	spreadPolicy := map[string]interface{}{
+		"erasure_lanes_enabled": policy.ErasureLanesEnabled,
+	}
+	if policy.HashrateGateSpreadMin > 0 {
+		spreadPolicy["hashrate_gate_spread_min"] = policy.HashrateGateSpreadMin
+	}
+	if policy.HashrateGateHPS > 0 {
+		spreadPolicy["hashrate_gate_hps"] = policy.HashrateGateHPS
+	}
+	raw, _ := json.Marshal(spreadPolicy)
+	return raw
+}