Computes a Valhalla isochrone per supplied location and streams the results as gzipped NDJSON.
Each line is one JSON object discriminated by type: a single header ({batch_id,total}),
then one result per location ({ref,index,features} on success, {ref,index,error,error_code}
on failure), then a terminating end ({succeeded,failed,truncated}).
A single point failing does not abort the stream. A missing end record means the response was
truncated and the results are incomplete.
Rejected with 400 if the selection exceeds the point cap or the travel-time budget.