ZipExecuteAdaptor dispatches only on the if_true branch #10908

Problem or motivation

ZipExecuteAdaptor and ZipReduceAdaptor (vortex-array/src/scalar_fn/fns/zip/kernel.rs) run a ZipKernel or ZipReduce only when the encoded array is child 0, the if_true branch. An encoded array in the if_false branch never reaches its kernel, so the zip decodes it even when the kernel could handle it. All seven ZipKernel implementations have this gap.

#10891 ran into it for Widen. Covering if_false needed a second hand-written ExecuteParentKernel and an enum to put the branches back in order, so the PR dropped the if_false case instead.

Proposed solution

Let the adaptors handle child 1, as CompareExecuteAdaptor does in vortex-array/src/scalar_fn/fns/binary/compare/mod.rs. That adaptor normalizes the child index so that the encoded array always comes first, and it swaps the operator.

Zip cannot simply swap its branches, because the mask selects if_true. Inverting the mask does not work either: a null mask row selects if_false, and not keeps it null. So ZipKernel::zip and ZipReduce::zip need to know which branch the array is, for example through a branch argument.

Additional context

Found in #10891 (tracking issue #10733). Once this lands, the Widen zip kernel can cover if_false with no extra code.