- Add stats_columns parameter to write_parquet_file trait
- Add stats_schema(), stats_columns(), get_clustering_columns() to Transaction
- Add stats_columns to WriteContext
- Update get_write_context() to take engine parameter
- Add clustering column support to expected_stats_schema()

- Add StatisticsCollector struct with new(), update(), finalize()
- Track numRecords across multiple RecordBatches
- Output StructArray with {numRecords, tightBounds}
- Basic unit tests for single/multiple batches

This is the foundation for full stats collection, adding column-level
stats (nullCount, minValues, maxValues) in subsequent PRs.

- Add null count tracking for all columns
- Support nested struct null counts
- Merge null counts across multiple batches
- Only collect for columns in stats_columns
- Tests for null counting across batches

- Add min/max tracking for all supported types
- Primitive types (int8-64, uint8-64, float32/64)
- Date, timestamp with all time units
- Decimal128
- String types with truncation to 32 chars
- Merge min/max across multiple batches
- Tests for min/max across single and multiple batches

- Add NullBuffer mask parameter to update()
- Only count masked-in rows for numRecords
- Only count nulls in masked-in rows for nullCount
- Filter column by mask before computing min/max
- Tests for mask behavior with min/max and null counting

This enables deletion vector support where masked-out rows
should not contribute to file statistics.