Segment-based (categorical) asset partitions for static sets — rollup (n-1) and fan-out (1-n)

[Lee-W]

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Segment-based (categorical) partitions — static sets

Categorical asset partitions whose key set is declared statically at authoring time (fixed regions / jurisdictions / known variants). Both cardinalities:

• n-1 (rollup) — fold a fixed segment set into one downstream run: RollupMapper(upstream_mapper=FixedKeyMapper("all_regions"), window=SegmentWindow(["us","eu","apac"])) (mapper/window primitives landed; release semantics via wait policies, Let partitioned Dag runs fire on a partial upstream window with wait_policy #66848.)

• 1-n (scatter) — fan one upstream event out to a fixed segment set: FanOutMapper(upstream_mapper=IdentityMapper(), window=SegmentWindow(["us","eu","apac"]), downstream_mapper=...)

SegmentWindow is the shared primitive for both directions; FixedKeyMapper supplies the rollup bucket. (Supersedes the earlier AllowedKeyMapper([...]) sketch.)

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