perf: use aligned slice access in SparkUnsafeArray bulk append

[andygrove]

Summary

• Replace per-element read_unaligned() + manual pointer arithmetic with slice-based indexed access in all nullable bulk append paths (impl_append_to_builder macro, append_booleans, append_timestamps, append_dates)
• Remove runtime alignment checks in non-nullable paths since alignment is guaranteed by SparkUnsafeArray layout, always use append_slice bulk copy
• Add debug_assert to verify the alignment invariant

Rationale

SparkUnsafeArray guarantees natural alignment for element data: the header is 8 + ceil(n/64)*8 bytes (always 8-byte aligned), and elements are at element_size stride from the aligned base. The nullable paths were previously doing per-element ptr.read_unaligned() with ptr = ptr.add(1), while the non-nullable paths had runtime alignment checks with fallback to unaligned reads. Since alignment is guaranteed, all paths can use slice access, which gives better codegen (no manual pointer arithmetic, compiler can reason about bounds).

Test plan

• cargo clippy --all-targets --workspace -- -D warnings passes
• Existing row-to-columnar and array element append tests/benchmarks cover these code paths