perf(variant): build BinaryView arrays directly

[cakeni]

Which issue does this PR close?

• Closes [Variant] Benchmark and optimize BinaryView construction in VariantArrayBuilder #10621.

Rationale for this change

Finalizing VariantArrayBuilder replayed every offset through BinaryViewBuilder::try_append_view, repeating bounds and value validation for buffers and offsets produced internally by the Variant builders.

What changes are included in this PR?

• Add a focused Criterion benchmark that times only build() for 262,144 small values.
• Construct views directly from the recorded offset slices.
• Build the final BinaryViewArray from those validated views, with the safety invariants documented at the unchecked constructor.

Are these changes tested?

• cargo +stable-x86_64-pc-windows-gnu test -p parquet-variant-compute --lib (347 passed)
• cargo +stable-x86_64-pc-windows-gnu bench -p parquet-variant-compute --bench variant_kernels -- variant_array_builder_build_262k_small_values --noplot
  • Before: [2.9213 ms, 2.9390 ms, 2.9577 ms]
  • After: [2.0393 ms, 2.0538 ms, 2.0689 ms]
  • Criterion change: [-30.816%, -30.118%, -29.428%], p = 0.00
• cargo fmt --all -- --check
• git diff --check

Are there any user-facing changes?

No API or behavior changes. VariantArrayBuilder::build is approximately 30% faster in the focused many-small-values benchmark.