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/erasure_lanes.go

@@ -0,0 +1,6 @@
+package config
+
+// ErasureLanesEnabled reports whether server policy allows multi-lane Reed–Solomon reassembly.
+func ErasureLanesEnabled(cfg RuntimeConfig) bool {
+	return cfg.ErasureLanesEnabled
+}