compute: columnar TopK input (C3)#37749
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Generalize `render_topk` to consume its input as a `CollectionEdge` instead of decoding to `Vec` up front. A new `map_topk_key` helper forms the TopK `(key, value)` arrangement input off the edge: the `Vec` arm maps the collection and reuses the owned row (behaviorally identical to before), and the columnar arm reads the batch via `into_index_iter`, forms the hash and group key from borrowed datums, and materializes the value row inline, so no `ColumnarToVec` decode sits on the input path. The three TopK variants (`build_topk`, `render_top1_monotonic`, and the inline MonotonicTopK path) route their key-forming through the helper. Temporal bucketing keeps its sanctioned leaf decode, and the rare column-limit error path stays byte-identical on the `Vec` edge via `into_vec` (the identity there). The intra-operator `explode_one`/`ensure_monotonic` on `Vec` are unchanged. Producers still emit `Vec` in Wave 1, so the columnar arm is dead in production and this adds no runtime change. Adds `map_topk_key_arms_agree`, a cross-arm unit test feeding the same input as a `Vec` and a columnar edge and asserting identical `(key, value)` updates across several timestamps. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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