Make PageReader AutoCloseable

PageReader implementations can own resources — ColumnChunkPageReader owns decompression / decryption buffers — but the interface has no standard teardown method. Today ColumnChunkPageReadStore.close() calls a package-private releaseBuffers() on the concrete class. Alternative PageReader implementations have no idiomatic way to declare that they own resources.

Have PageReader extend AutoCloseable with a default no-op close(). Rename ColumnChunkPageReader.releaseBuffers() to close().

Start with the PageReader interface, then inspect ColumnChunkPageReader and ColumnChunkPageReadStore, especially the existing releaseBuffers() call. Make the interface resource-safe with a default teardown and ensure the concrete reader and store use the renamed close() method; run the relevant reader and page-store tests to confirm resource cleanup still works.

Written by the indexing model from the issue text.