[alexanderbianchi]

Eager Iceberg scans currently retain files inside a work-unit feed. Before routing, DFD converts that feed into a remote handle, so the existing routing callback cannot inspect the assigned files.

This draft explores representing eager assignments directly in the scan, while retaining feeds for lazy discovery:

pub enum ScanInput {
    AssignedFiles(AssignedFiles),
    WorkUnitFeed(Box<WorkUnitFeed<IcebergWorkUnitFeed>>),
}

Eager: discover files → choose task count → assign file groups
                                              ↓
                                   task-specific Iceberg scan
                                      ↙               ↘
                              routing inspects     worker reads

Lazy: scan definition → discover during execution → feed → worker reads

Mechanics

• One IcebergDataSource and reader handle both inputs. AssignedFiles exposes borrowed files grouped by local partition.
• The existing scale-up hook constructs DistributedLeafExec variants after the final task count is known. Each variant owns a concrete assignment; routing and execution no longer independently calculate it.
• Coordinator variants share retained descriptors and their memory reservation. The codec sends only the selected task's groups and preserves their partition membership on the worker.
• The existing RouteTaskEvent::task_specialized_plan exposes these eager groups directly. No core DFD routing changes, additional plan view, affinity hints, or placement policy are introduced.
• Lazy feeds retain their existing execution-time discovery and round-robin delivery. Eager scans retain the current choice between round-robin and greedy assignment; this draft does not make greedy mandatory.
• Whole-scan statistics remain on the original planning scan. Task variants report their own file/row totals and omit whole-scan column aggregates that would describe the wrong subset.

Stacked on #789. This is an alternative to #797 and #790, intended to compare explicit assigned work with exposing coordinator-local plans to routing. It does not add ordered-read strategies.

Tradeoffs and compatibility

Eager plans carry file descriptors, increasing plan payload size and moving their transport to worker setup. Greedy assignment runs synchronously in the existing scale-up hook. No performance improvement is claimed or benchmarked.

IcebergDataSource::feed() now returns Option; eager callers inspect assigned_files() or input(). Coordinators and workers must be upgraded together before using the new eager wire representation. Lazy feed plans keep their existing representation. The upgrade guide documents both changes.

Validation

• Full Iceberg unit, integration, and documentation tests passed.
• Routing tests cover eager round-robin/greedy, lazy feeds, isolated UNION branches, exact serialized file groups, and repeated execution before and after serialization.
• Existing distribution tests continue to check per-task row counts across the new scan-variant layout.
• Formatting and all-target Iceberg Clippy checks passed with warnings denied.