[alexanderbianchi]

Eager scans as standard file scans

This draft explores an alternative to #799: one Iceberg FileScanTask is one PartitionedFile in a DataFusion FileScanConfig. The file entry exposes path and size, while its typed IcebergFileTask extension retains the complete Iceberg descriptor, including deletes and residual predicates.

Before:
  eager planning -> retained files behind a feed
  task plan      -> feed ID
  execution      -> file descriptors arrive through feed messages

This draft:
  eager planning -> FileScanConfig.file_groups
  task plan      -> assigned file groups + Iceberg task metadata
  execution      -> IcebergFileSource opens each task using iceberg-rust

The existing routing callback can inspect the task's file groups before choosing a worker. There is no ScanInput enum, separate AssignedFiles grouping, second routing plan, or core DFD API change. Lazy scans retain their existing feed-backed source.

What remains Iceberg-specific

• Manifest planning, pruning, and selected-file statistics.
• Round-robin/greedy assignment to task-local file groups.
• An Iceberg file opener that retains the existing reader's delete and schema semantics.
• Codec support for per-file metadata: DataFusion's scan-config codec is reused, but it does not serialize arbitrary file extensions.
• Whole-task assignment; automatic byte-range splitting is disabled.

Planning statistics are transferred to the initial file scan. Task variants use unknown statistics rather than incorrectly reusing whole-scan aggregates.

Questions this experiment exposes

• DataFusion now schedules individual file opens. The Iceberg reader receives one task per open, so iceberg.data_file_concurrency_limit no longer controls concurrency across eager files. No performance claim is made.
• The opener still uses Iceberg FileIO. The config's local object-store URL is an unused placeholder; storage-aware routing should use the extension's original complete URI.
• Assignment runs synchronously in DFD's scale-up callback.
• Re-execution follows DataFusion's reset_state() contract for file scans.
• Coordinators and workers must be upgraded together for the new eager payload.

Stacked on #789, independently of #797 and #799, so the alternatives remain reviewable separately.

Validation

• Full Iceberg suite: 70 tests and 1 doctest passed.
• Formatting and Clippy (all targets, warnings denied) passed.
• Routing tests compare full serialized task descriptors before/after codec round-trips and execute both assignment policies, isolated UNION branches, and reset scans.