fix: harden supervisor recovery and stuck scan

[liobrasil]

Summary

• add the final supervisor regression coverage for duplicate recoveries and mixed recurring/non-recurring scan sets
• fix duplicate recovery churn by treating in-flight executed recurring transactions as active until rebalance state advances
• restrict stuck-scan ordering to recurring participants and lazily prune stale non-recurring entries during candidate walks
• align scheduler docs/comments with the recurring-only scan behavior

Verification

• flow test cadence/tests/scheduler_mixed_population_regression_test.cdc
• flow test cadence/tests/scheduled_supervisor_test.cdc