fix: resolve critical memory leak and renderer undefined errors

- Fix memory leak in VPivottableUi by implementing proper PivotData memoization
- Replace inefficient watchEffect with controlled recreation based on structure changes
- Add deep cleanup to break circular references between PivotData and aggregators
- Fix renderer undefined error (Issue #269) by adding default TableRenderer
- Add null safety check for renderer access
- Optimize PivotData creation to only occur when essential properties change

Fixes #269, #270

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: Seungwoo321