Header whitelist in a coordinator process silently drops new fields added in worker replies

When a multi-process helper (coordinator + workers) builds its reply header by spreading an explicit list of fields from each worker result (e.g. ...(r.text ? {text, kind} : {})), any new field the native addon and worker now return (here a per-signal mean text size) never reaches the client. The tests pass at the addon and worker level and fail only end-to-end with the field undefined. When adding a reply field, grep the coordinator for where the header is assembled. Also: JSON.stringify drops keys whose value is undefined, so a missing field shows as absent, not as null, which makes this easy to miss in logs.