Larger Context

This ticket adds switch_role from WT-18868 to probability-based workload generation. It builds on the follower-mode foundation from WT-18867 and can adopt verified relaxations from WT-18869.

Motivation

Randomised workloads are required to exercise repeated role transitions with transactions, timestamps, checkpoints, crashes, and restarts.

Scope

Add a switch_role probability and generate workloads with zero, one, or multiple transitions. Track the current role while building the serial-equivalent workload. Reuse the sequence dependency graph to make each synchronous transition a full transaction barrier, and extend timestamp-block assignment for role-specific stable and checkpoint preparation.

Keep crash and restart generation enabled in both roles. Add counterexample-reduction tests, preserve deterministic replay through expanded workload files and recorded seeds, and add one Evergreen task named model-test-switch.

Acceptance Criteria

• Seeded generation produces valid workloads with zero, one, and multiple role transitions.
• Transition barriers, timestamps, crashes, and restarts behave correctly in both roles.
• Counterexample reduction completes with valid switch_role workloads.
• Model and WiredTiger return codes and final recovered data match.
• model-test-switch runs the randomised workload in Evergreen.

Outcome

Evergreen continuously runs reproducible and reducible randomised single-node switch-mode workloads.