Retention: take max across shared aggregation refs; sliding depth (n-1)*W/S+1 #760

Problem

Shared circular-buffer aggregation references previously used the last num_aggregates_to_retain value, so a later short-range query could reduce a shared aggregation's configured capacity.

Decision

Optimizer output uses ReadBased cleanup. It emits and aggregates read_count_threshold; it does not configure num_aggregates_to_retain. Therefore the CircularBuffer-specific sliding depth formula (n−1)·W/S + 1 does not apply to the optimizer path. If optimizer-selectable CircularBuffer cleanup is introduced later, its exact sliding retention calculation should be a separate issue.

Change

• Shared CircularBuffer references retain the maximum num_aggregates_to_retain.
• Shared ReadBased references sum read_count_threshold.
• Optimizer output activates ReadBased cleanup for both value and paired key aggregations.

Part of #753.