Summary

Removes implicit rebuilds from ListViewArray take and filter compute kernels, adds density-introspection methods to support deciding when to rebuild explicitly at materialization boundaries, and defers rebuilding until array export.

Motivation. The previous code rebuilt the elements buffer eagerly on every take or filter whose row-fraction dropped below REBUILD_DENSITY_THRESHOLD. In an execution tree like take → take → ..., an eager mid-pipeline rebuild costs an allocation and a full copy of referenced ranges that the next operator may immediately sparsify away.

The row-fraction heuristic was also inaccurate. It doesn't account for per-row size variance, unreferenced elements, and duplicate references.

Changes:

• Drop implicit rebuild from TakeReduce::take and TakeExecute::take
• Drop implicit rebuild from filter_listview
• Add methods to calculate reference density: compute_referenced_elements_mask, compute_density, and estimate_density
• Add ListViewArray::should_rebuild(exact: bool): Currently unused but intended to gate an explicit compaction boundary in follow-up.

API Changes

take/filter over a sparse or low-density ListViewArray no longer allocates a compacted elements buffer. Downstream consumers must call rebuild and should_rebuild explicitly themselves.

Testing

• New vortex-array/src/arrays/listview/tests/density.rs with multiple tests: