…ecked reads

Profiling and disassembly of the gather inner loop showed the per-bit cost was
dominated not by the 64-bit packing (optimal for a random gather) but by the
overhead around each read: a wide `&[BitBuffer]` struct index, the redundant
bounds `assert` inside `BitBuffer::value`, and on the nullable path a per-row
`Option`/`Mask`-variant dispatch.

Pre-validate the per-row bounds once, then hoist a raw `(ptr, bit_offset)` per
value buffer and read each selected bit with `get_bit_unchecked`. Materialize
each branch's validity into one full-length `BitBuffer` so the validity gather
is the same uniform unchecked read. Measured ~1.2x faster non-nullable and
~1.4x nullable across random / round-robin / single-branch patterns.

Also adds two two-value word-boundary regression tests exercising the gather
across more than one 64-bit packing word, in the non-nullable and nullable
cases, validated against the existing scalar reference oracle.

Signed-off-by: Joe Isaacs <joe.isaacs@live.co.uk>

Remove the arrow-rs `interleave` comparison arm and its input builder, keeping
the Vortex-only benchmark across the random / round_robin / single_branch
access patterns (N=2, N=4, nullable and non-nullable). No dependency changes:
every arrow crate in vortex-array is used by the crate's own source.

Signed-off-by: Joe Isaacs <joe.isaacs@live.co.uk>

…ctor types

Drop the `AsPrimitive<usize>` bound on the bool gather: dispatch both selectors
over their concrete unsigned width via `match_each_unsigned_integer_ptype!` and
convert to an index with a plain `as usize` on the native type, passed in as
`branch_at` / `row_at` accessors. Keeps the selector loads at their native width.

Signed-off-by: Joe Isaacs <joe.isaacs@live.co.uk>

Keep only 2-child round-robin, 2-child random, and 64-child random (each
nullable and non-nullable); drop the single_branch pattern and the N=4 variants.

Signed-off-by: Joe Isaacs <joe.isaacs@live.co.uk>

…> selectors

Go back to the generic `gather<A: AsPrimitive<usize>, R: AsPrimitive<usize>>`
with `.as_()` conversions, dropping the per-type `as usize` accessors. The
trait conversion avoids the `cast_possible_truncation` lint without a scoped
allow and matches the idiom used by the other bool kernels (e.g. take).

Signed-off-by: Joe Isaacs <joe.isaacs@live.co.uk>

…tern

Replace `Combo.pattern: &'static str` and the `match`/`unreachable!` on string
literals with a `Pattern` enum (RoundRobin, Random), making the match exhaustive
and dropping the unreachable arms.

Signed-off-by: Joe Isaacs <joe.isaacs@live.co.uk>

…lidity

Validity is just another set of boolean buffers routed by the same selectors, so
factor the hoisted-pointer unchecked gather into `gather_bits` and call it for
both the values and the materialized per-branch validity, instead of duplicating
the pointer-table build and `collect_bool` loop.

Signed-off-by: Joe Isaacs <joe.isaacs@live.co.uk>

Signed-off-by: Joe Isaacs <joe.isaacs@live.co.uk>

Signed-off-by: Joe Isaacs <joe.isaacs@live.co.uk>

…tructor docs

The `new_unchecked` / `new_unchecked_slots` doc comments intra-doc-linked
`[`Interleave::check`]`, a private item, failing `rustdoc::private_intra_doc_links`
(`-D warnings`). Drop the link brackets, keeping the inline-code reference.

Signed-off-by: Joe Isaacs <joe.isaacs@live.co.uk>