docs(audits): profile Arc<Mutex<Vec>> contention plan (#1192)

[oferchen]

Summary

• Static audit of every Arc<Mutex<Vec<...>>> and Mutex<Vec<...>> site reachable from parallel-stat / drain_parallel hot paths in transfer, engine, and flist.
• Measurement plan covering perf lock:contention_*, thread-sweep perf stat, and criterion benches over BatchedStatCache and drain_parallel.
• Decision criteria gating any replacement: wall-clock, lock contention, RSS, and unsafe-policy / dep-budget review.

Test plan

• Documentation-only change; no code modified.
• CI doc / workspace checks pass.