Adds zoom as an optional third parameter to the reduce callback

[efreethy]

Adds zoom as an optional third parameter to the reduce callback function, enabling zoom-aware aggregation of cluster properties.

Use Case

This is useful when users need to track how properties aggregate differently at different zoom levels. For example:

• Debugging cluster aggregation behavior across zoom levels
• Creating custom visualizations that need zoom-level awareness
• Implementing zoom-dependent weighting in cluster calculations

Changes

• Modified reduce callback signature from (accumulated, props) to (accumulated, props, zoom)
• Updated JSDoc comment to reflect the new parameter
• Added test case to verify zoom is correctly passed to reduce function

Backward Compatibility

This change is fully backward compatible - existing reduce functions that don't use the zoom parameter will continue to work unchanged.

Testing

All existing tests pass, plus new test verifying zoom parameter is passed correctly.