compute: columnar FlatMap input, fueling preserved - #37758
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Rework `render_flat_map` so the bespoke `FlatMapStage` consumes the input `CollectionEdge` directly instead of decoding it via `as_specific_collection` (which forces `into_vec`). The unarranged path reads the edge, so the columnar arm never decodes rows at the input; the keyed path still reads an existing arrangement, already columnar internally, presented as a `Vec` edge. The fuel machinery is the point of this operator: it bounds how much work a single `generate_series` can do before yielding the worker. To keep the `Vec` and columnar arms from drifting, the fuel queue, per-activation budget, and re-activation now live in one generic `flat_map_stage` function. The only arm-specific piece is how records are read from a queued batch, factored into a small `FlatMapBatch` trait: the `Vec` arm drains owned rows, the columnar arm iterates the borrowed column via `into_index_iter` and never materializes an owned `Row`. The per-record table-function expansion and mfp drain are shared in `process_flat_map_row`. FlatMap is explicitly not routed through `flat_map_datums`, which drains a full batch with no fuel budget. Producers still emit `Vec`, so the columnar arm is dead in production and this carries no runtime change. Two unit tests: `flat_map_fuel_bounds_per_activation` proves a small fuel budget spreads a multi-batch input across multiple activations while an unbounded budget drains it in one, and `flat_map_arms_agree` proves the `Vec` and columnar arms produce identical output across multiple timestamps including a retraction. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
A per-record `inspect` needs `&Container: IntoIterator`, and on macOS resolving that recurses through `objc2`'s blanket impls until the trait solver overflows, so `lint-macos` fails to compile the test. Counting through the container avoids the resolution entirely. The same test reads the container directly from the commit that makes FlatMap's output columnar onward, so this only closes the window where it did not.
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FlatMap reads its input from the columnar edge, preserving the existing fuel and backpressure behavior through a generic fueled stage operator.
Columnar dataflow-edge migration. Design doc:
doc/developer/design/20260720_columnar_dataflow_edges.md(#37744).Part of CPU-51.