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_test.go

@@ -0,0 +1,12 @@
+package config
+
+import "testing"
+
+func TestErasureLanesEnabled(t *testing.T) {
+	if ErasureLanesEnabled(RuntimeConfig{}) {
+		t.Fatal("expected off by default")
+	}
+	if !ErasureLanesEnabled(RuntimeConfig{BuiltinConfig: BuiltinConfig{ErasureLanesEnabled: true}}) {
+		t.Fatal("expected on when set")
+	}
+}