perf: vectorize small u8 table take with NEON

[joseph-isaacs]

Rationale

Low-cardinality dictionaries with u8 codes and one-byte values can decode through an in-register NEON table instead of arbitrary scalar lookups.

Stacked on benchmark baseline #9566. The x86 implementation is intentionally split into the next PR.

Changes

• Use NEON TBL on little-endian AArch64.
• Apply the path to u8 codes, at most 16 one-byte values, and at least 64 rows.
• Retain bounds checks and the existing fallback.
• Add correctness and out-of-bounds tests.

CodSpeed wall-time results

Medians over 1,000 samples on Graviton3 metal, compared with #9566:

Rows	Baseline	NEON	Speedup	Time reduction
1M	552.4 µs	51.06 µs	10.82×	90.8%
16M	8.786 ms	799.7 µs	10.99×	90.9%

Runs: baseline, optimized.

Checks

• focused small-table correctness and bounds tests
• targeted Clippy with warnings denied
• cargo +nightly fmt --all --check
• git diff --check