[BongHwi]

GroupedCombinations currently constructs typed associated slices before the consumer can check whether they are empty. For sparse associations, repeatedly constructing these slices can dominate the mixing workload, even when the consumer immediately discards empty tuples.

This change adds an explicit GroupedSlicePolicy::SkipEmpty option. It checks association counts through the existing sorted SliceCache before constructing typed slices.

• IncludeEmpty remains the default, preserving the existing yielded stream.
• Skipped tuples are traversed using the original grouping policy. Categories, mixing depth, partner direction and window boundaries are preserved.
• Window metadata retains its original-policy meaning.
• The opt-in accepts raw associated tables and retains the existing sorted-cache and lifetime requirements.

Consumers should enable this option only when processing an empty-associated tuple has no observable effects, including normalization, counters or window bookkeeping.

Performance

A three-run evaluation of an Omega2012 mixed-event workload on macOS arm64, using a fixed seven-DF input and mixing depth 10, gave the following mean CPU times:

Mode	Baseline CPU	SkipEmpty CPU	Reduction
Xi, K0s	235.26 s	5.39 s	97.71%
Xi1530, K	151.54 s	5.63 s	96.28%

These are complete local DPL CPU times (user + system, including child processes). All valid measurements were included. The Xi, K0s baseline had a coefficient of variation of 5.19%.

The input has sparse cascade associations, so these gains are workload-dependent. Dense inputs may benefit less or incur additional cache-lookup overhead.

Validation

• Added Arrow-backed integration coverage for Pair/Triple policies, default behavior, iterator copies, bindings, filtered grouping, cache refresh and original window metadata.
• Rebuilt and ran the new integration scenarios together with related ASoA, ASoAHelpers and GroupSlicer coverage: 50 cases passed.
• Separately validated the current-source Omega2012 opt-ins across twelve full-input executions, covering same-event, mixed-event and combined processing for both modes.
• All six baseline-versus-opt-in ROOT-output comparisons passed without numerical tolerances.

[BongHwi]

This improves a lot for the event-mixing when we handle the very rare daughter case, such as v0 and cascade.