ENH: Optimize LUT with transposed loads and 4-way row unrolling (#7)

This patch add several improvements to the Lookup Table (LUT) for non-scalable architectures.

- **Transposed loads:** Introduced a pre-calculated transposed storage (`trans_`) and loading strategy.
   This allows using optimized interleaved loads (X2 and X4) when vector lanes match the blocking factor,
   reducing the need for expensive gather operations.
- **FourTablesLookup:** Handle case where the vector length is exactly one-quarter
   of the table width using 4-way table lookups.

Maintenance:
- Removed runtime specializations for scalable extensions.
  These now explicitly fallback to Highway's `GatherIndex` (no
  performance gain on SVE).

Author: seiko2plus