For-loop pipeline producers buffer every iteration before downstream consumption

Follow-up to #453: for f in /data/*.json; do jq -r .id "$f"; done | head -n 1 executes all loop iterations before head can consume output. Streaming jq alone cannot fix the outer interpreter buffer.

Proposed scope: a first-stage for producer with static word expansion and no outer redirections emits completed body-statement output through the existing bounded pipeline. Keep body execution/redirects and loop state isolated, preserve complete output and break/continue semantics, and stop on downstream closure. Other compound forms remain unchanged.