feat(transfer): add ReorderBuffer stall-duration and queue-depth metrics

[oferchen]

Summary

• Add ReorderBufferStats struct with five counters: current_depth, peak_depth, stall_count, total_stall_nanos, and items_delivered to the BoundedReorderBuffer in the transfer crate
• Instrument insert() to track stall episodes (head-of-line blocking when a non-head item arrives into an empty buffer) and queue depth high-water marks
• Add metrics() accessor returning a snapshot of all counters, plus mean_stall_nanos() derived accessor
• Support deterministic clock injection via with_clock() constructor for reproducible test assertions on stall timing

Implements the observability layer from the design note in docs/design/reorderbuffer-metrics-and-bypass.md (Part A). This is a prerequisite for the bypass optimization (#1886) - the metrics confirm whether head-of-line stalls are significant enough to justify bypassing the reorder buffer when --delay-updates is off.

Test plan

• in_order_delivery_no_stall - verifies zero stalls and zero stall time for sequential insertion
• out_of_order_produces_stall - verifies stall count increments and stall duration accumulates when head is missing
• multiple_stall_episodes - verifies distinct stall episodes are counted independently
• peak_depth_monotonically_nondecreasing - verifies peak depth never decreases across insertions and drains
• queue_depth_tracks_buffered_count - verifies current_depth matches buffered_count()
• items_delivered_counts_all_drained - verifies delivery counter accuracy
• mean_stall_nanos_derived_correctly - verifies arithmetic on the derived accessor
• stale_insert_does_not_affect_metrics - verifies already-delivered sequence numbers do not perturb counters
• Property test: counters_monotonic_across_permutation - all counters are monotonically non-decreasing across reversed permutations
• Property test: identity_permutation_zero_stalls - identity permutation always produces zero stalls
• All 25 existing reorder buffer tests continue to pass