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.

agent/client/erasure_policy_test.go

@@ -0,0 +1,17 @@
+package client
+
+import (
+	"testing"
+)
+
+func TestApplySpreadPolicyErasureLanes(t *testing.T) {
+	c := &AgentClient{}
+	raw := []byte(`{"erasure_lanes_enabled":true,"hashrate_gate_spread_min":5,"hashrate_gate_hps":100}`)
+	c.applySpreadPolicyJSON(raw)
+	if !c.cfg.ErasureLanesEnabled {
+		t.Fatal("expected erasure_lanes_enabled")
+	}
+	if c.cfg.HashrateGateSpreadMin != 5 {
+		t.Fatalf("min=%d", c.cfg.HashrateGateSpreadMin)
+	}
+}