compute: columnar FlatMap input, fueling preserved (C2)

[antiguru]

Rework render_flat_map so the bespoke FlatMapStage consumes the input CollectionEdge directly instead of decoding 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.
The only arm-specific piece is reading records 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.
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.

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; flat_map_arms_agree proves the two arms produce identical output across multiple timestamps including a retraction.