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.

crates/irtt-stats/Cargo.toml

@@ -13,3 +13,6 @@ categories = ["network-programming"]
 
 [dependencies]
 irtt-client.workspace = true
+
+[dev-dependencies]
+proptest = "1.11.0"