engine: the audit trail grows with every task of every loop sweep, even in an untraced run #373

Summary

Every executed task pushes an AuditTrail entry onto message.audit_trail (dataflow-rs 3.14.0 workflow_executor.rs:2518), and nothing prunes it — including in an untraced run (task_details: false, capture_changes off), where nothing reads it before the run ends.

In a loop workflow that is O(tasks × sweeps) entries held for the life of the run: a 1000-sweep turn loop with ~7 + 2·seats tasks per sweep holds ~23k entries at 8 seats. Each is small (two Arc<str> and a timestamp), but it is also deep-cloned into every for_each element (#372), so it multiplies.

Proposal

When the run is untraced, keep a bounded audit trail (e.g. the last N entries, or counts only), or none; or clear the per-sweep entries at each loop sweep the way scratch is reset. #350 made capture opt-in for the same reason — the audit trail is the remaining per-sweep growth in an untraced loop.