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

[nvdtf]

This is the same issue we faced in onflow/FlowYieldVaultsEVM#70
The problem with the fix is that if the transaction panics, lastRebalanceTimestamp is not updated. This makes the rebalancer permanantely stuck because it's Executed and the lastRebalanceTimestamp was never updated.
You might want to consider a grace period based fix.