CPU work done on IO runtime #3589

Apache Iceberg Rust version

v1.94.0

Describe the bug

Parquet decoding is not consistently isolated from I/O work when using separate CPU and I/O runtimes:

• Data-file footer and page-index decoding run on the I/O runtime.
• Data-file page decoding runs on whichever runtime polls the scan stream, which may be the I/O runtime.
• Delete-file footer parsing, page decoding, and delete-filter construction run on the I/O runtime.

This allows CPU-intensive decoding and decompression to compete with storage requests on I/O workers.

Expected behavior: perform Parquet decoding and batch processing on the configured CPU runtime, while keeping storage opens and byte reads on the I/O runtime. Preserve concurrency limits, backpressure, cancellation, and error propagation.

Expected behavior

Decoding work should be done in the CPU runtime.

Willingness to contribute

Draft PRs:

• [#3587 — Decode delete files on the CPU runtime](fix(arrow): decode delete files on the CPU runtime #3587)
• [#3588 — Decode data files on the CPU runtime](fix(arrow): decode data files on the CPU runtime #3588), dependent on fix(arrow): decode delete files on the CPU runtime #3587.