pub const APPEND_META_ENV: &str = "XS_APPEND_META";Expand description
Env var carrying the base metadata stamped on every frame a runner appends
(service_id, {action_id, frame_id}, …), as a JSON object string.
Injecting it per instance keeps .append instance-independent, so one decl
can be registered on a prepared engine and reused across spawns and restarts.