feat: add batch reader infrastructure

Changes

BatchReader

• Abstract base class for reading batch data into Arrow arrays. Supports NextBatch() returning ArrowArray + ArrowSchema, and NextBatchWithBitmap() returning batches with a validity bitmap for efficient deletion vector and index filtering. Provides EOF detection and reader metrics.

FileBatchReader

• Extends BatchReader for single-file reading scenarios. Adds SetReadSchema() for dynamic schema/predicate reconfiguration, GetFileSchema(), GetPreviousBatchFirstRowNumber(), GetNumberOfRows(), and SupportPreciseBitmapSelection() for bitmap-based row selection.

PrefetchFileBatchReader

• Extends FileBatchReader with prefetch capabilities.

CompleteRowKindBatchReader

• BatchReader wrapper that prepends a _VALUE_KIND column (int8 array of RowKind values) to each output batch.

ConcatBatchReader

• Concatenates a list of BatchReader instances and reads them sequentially.

DelegatingPrefetchReader

• FileBatchReader adapter that wraps a PrefetchFileBatchReaderImpl. Transparently delegates to the prefetch reader or its first underlying reader based on whether prefetching is needed.

PredicateBatchReader

• BatchReader wrapper that applies a Predicate filter to each batch.

ReaderUtils

• Static utility class.

Tests

• CompleteRowKindBatchReaderTest
• ConcatBatchReaderTest
• PredicateBatchReaderTest
• ReaderUtilsTest