docs(audits): Arc<Mutex<Vec>> contention under parallel stat static analysis (#1192)

[oferchen]

Summary

• Static analysis of every remaining Arc<Mutex<Vec<...>>> in the workspace, focused on the parallel stat hot path used by the receiver (candidates.rs:117-132) and the sender file-list walker (walk.rs:262-267).
• Confirms parallel stat does not allocate any shared mutex-guarded accumulator: map_blocking (crates/transfer/src/parallel_io.rs:107-125) delegates to rayon's into_par_iter().map().collect(). The three remaining production sites are SSH stderr drain (aux_channel.rs:99,148) and drain_parallel shards (drain.rs:63-64); only the latter is on a rayon hot path and is sharded already.
• Cites the just-merged drain-side audit (PR docs(audits): static analysis of drain_parallel Mutex contention #3699), scopes Normalize burst handling for unlimited bandwidth limits #1192 to the runtime profiling needed for closure, and lists mitigations (per-thread accumulator, lock-free MPSC, DashMap, no-op) to evaluate only if profiling surfaces a residual Mutex<Vec>.

Test plan

• Reviewer skims the inventory table and confirms it matches a fresh rg "Arc<Mutex<Vec" workspace search.
• Reviewer cross-checks file:LINE citations against the listed source paths.
• Reviewer verifies the audit cites PR docs(audits): static analysis of drain_parallel Mutex contention #3699 for the drain-side analysis rather than restating it.