perf: speed up Utf8View/BinaryView hashing

Specialize the Utf8View/BinaryView hash dispatch:

- Inlined-only paths (no external buffers) get an unrolled u128 hot loop
  for fresh and rehash. Lane widths are narrower than the primitive
  variants (8-way fresh, 4-way rehash) to avoid spilling u128 lanes.
- Nullable inlined-only paths walk the null bitmap in u64 chunks and
  branchlessly csel-blend mixed chunks, instead of paying per-row
  `is_null`.
- Buffered paths (external data buffers) keep the per-row loop since
  buffer-load latency dominates the null check.

Measured on M4 Max, BATCH_SIZE=8192, 3% null density:

| case                                | before    | after     | Δ      |
|-------------------------------------|-----------|-----------|--------|
| utf8_view: single, no nulls         | 19.11 µs  | 17.23 µs  | -9.8%  |
| utf8_view: multiple, no nulls (3)   | 54.13 µs  | 53.31 µs  | -1.5%  |
| utf8_view: single, nulls            | 19.62 µs  | 19.54 µs  | -0.4%  |
| utf8_view: multiple, nulls (3)      | 58.86 µs  | 58.03 µs  | -1.4%  |
| utf8_view (small): single, no nulls | 2.55 µs   | 2.25 µs   | -11.6% |
| utf8_view (small): multiple, no    | 9.77 µs   | 8.47 µs   | -13.3% |
| utf8_view (small): single, nulls   | 6.91 µs   | 3.65 µs   | -47.2% |
| utf8_view (small): multiple, nulls | 20.43 µs  | 10.18 µs  | -50.2% |

Author: Rich-T-kid