test(stats): add rolling-window and IPDV property coverage

Add proptest-based generative tests for irtt-stats's stateful rolling
windows and IPDV pairing.

The rolling_count and rolling_time bounds turn out to be two
independent VecDeques (rolling.rs), not a single window bounded by
both, so the main property test proves that independence: it drives a
StatsCollector configured with both bounds simultaneously through
generated operation streams, checking after every operation that each
window matches a from-scratch reference model applying only its own
bound (and that the cumulative snapshot matches a full-history
replay). Since both the production window and the reference replay
the same events through the same deterministic aggregation code, the
comparison is exact equality, not a tolerance.

Also add two smaller property tests for IpdvTracker's bounded FIFO
state and at-most-once adjacent-pair emission under arbitrary
(including non-monotonic and duplicate) sequence streams, complementing
the existing strictly-increasing hand test.

No discrepancies between production and the reference model were
found.

Cargo.lock

@@ -778,6 +778,7 @@ name = "irtt-stats"
 version = "0.5.0"
 dependencies = [
  "irtt-client",
+ "proptest",
 ]
 
 [[package]]