Bound the sorted-scan merge by how many files' key ranges overlap #71

With iceberg.execution.preserve_data_ordering, #55 merges one reader per data file with every file open at once, so iceberg.execution.max_merge_files_per_partition caps the number of files. A sorted table with many small commits goes over the cap and loses its order, even when most of its files don't overlap.

The plan is to bound the merge by overlap instead of file count. IcebergDataSource would read each file's bounds and null counts for the sort columns, sort the files by their first key in output order, and put each file after the first chain whose last file ends at or before it. DataFusion's split_groups_by_statistics does the same. Each chain is read one file at a time, the chains are k-way merged, and max_merge_files applies to the number of chains, which equals the maximum overlap depth. A table whose files don't overlap then merges nothing.

Cases that need care:

• Files are compared over every claimed sort key, not just the leading one. With a tie on the leading key, comparing only that key could put one file's rows ahead of the other's when the second key says otherwise.
• Under NULLS FIRST a file with nulls can only start a chain, and under NULLS LAST it can only end one. A missing null count on an optional column counts as nulls.
• Descending keys swap the bounds.
• Truncated string bounds stay correct; they just overlap more.
• A file with missing bounds, or a bound typed differently from the column after a type promotion, gets a chain of its own. With no bounds at all, the scan behaves exactly as after feat: report the sort order of sorted Iceberg scans to DataFusion #55.
• Listing every file at planning time also needs a cap of its own.

This needs per-file column statistics on FileScanTask, which iceberg-rust doesn't carry today. That change is apache/iceberg-rust#3422, in PR apache/iceberg-rust#3424. Once it merges, this needs a bump of the iceberg-rust rev.

Related: In apache/datafusion-comet#5331 (comment), @mbutrovich proposed admitting unopened files from a priority queue ordered by min bound. That bounds open readers by the same overlap depth and opens files later, which helps LIMIT, but it needs a custom merge operator. Chains reuse StreamingMergeBuilder, so I'd start there and keep admission as a possible next step.

I'm working on this and will put up the PR here once #55 and the iceberg-rust change have merged.