test(client): model SessionMachine open-session operation sequences

Adds a proptest-based model test that drives generated sequences of
probe sends, valid/duplicate/late/unknown/wrong-token replies, bounded
and unbounded timeout polling, peer/local close, and u32 wire-sequence
wraparound through a real SessionMachine and an independent reference
model, asserting the observable state and every emitted event match
after each operation. This combinatorial interleaving of an open
session's operations was previously untested; existing tests are all
narrow single-scenario cases.

Cargo.lock

@@ -735,6 +735,7 @@ dependencies = [
  "irtt-proto",
  "irtt-server",
  "nix 0.31.3",
+ "proptest",
  "socket2",
  "thiserror 2.0.20",
  "tokio",