Native Iceberg sorted-merge scan rebuilds the delete-file cache per file (shared deletes re-read) #6524

Describe the bug

Native Iceberg reader (native/core/src/execution/operators/iceberg_scan.rs) - on the sorted-merge read path added in #5331, IcebergScanExec is multi-partition — one file
per Spark partition, one execute() call per file, with a SortPreservingMergeExec above. Each
execute_with_tasks builds a fresh ArrowReaderBuilder(...).build(), and that is where
iceberg-rust constructs the CachingDeleteFileLoader. Its delete_filter is the shared state the
crate documents as existing "to allow caching loaded deletes across multiple calls to
load_deletes (e.g., across multiple file scan tasks)."

Because the reader is rebuilt per file, that cache is now per-file. A delete file that applies to
the whole partition is downloaded and parsed once per data file instead of once for the partition.
fill_delete_file_sizes de-dups only within a single call, so it also issues one HEAD per data
file for the same delete file.

Rows are correct either way — this is purely I/O and CPU, not a correctness problem.

Measurement

Two data files sharing one positional delete file:

• Unordered path: bytes_scanned = 2717
• Ordered (merge) path: bytes_scanned = 4256

The 1539-byte delta is exactly the delete file being read a second time. At the default cap
(spark.comet.scan.icebergNative.sortMerge.maxFilesPerPartition = 64) this is up to ~64x on a
merge-on-read table, and equality deletes are the worst case since they are always partition-scoped
and the expensive ones to parse.

Suggested fix

Same as in file_io : hold one ArrowReader onIcebergScanExec and clone it per partition so the CachingDeleteFileLoader (and its delete_filter) is shared across the partition's files. It needs batch_size off the TaskContext, so it has to be a OnceLock filled on first execute rather than built in new.
ArrowReader is Clone, and read() calls ScanMetrics::new() per invocation and re-bases the
loader's metrics through with_scan_metrics, so per-partition metrics stay separate while the
delete cache is shared. With that prototype the ordered path drops back to bytes_scanned = 2717
with identical rows.

Additional context

• Reported by @andygrove during review of feat: Sort-aware Iceberg reads in Comet via a per-partition streaming merge #5331.
• Distinct from Bound memory when a partition has many files #5343 (which is about bounding how many files the merge opens at once); this is
  about the delete-loader being rebuilt per execute. Filing separately so the regression shape and
  the measurement are not lost.