perf: optimize spark_unhex in spark-expr

[andygrove]

⚠️ Autonomously generated by an LLM. This PR was created end-to-end by an automated agent with no human author. It has passed automated correctness (unit tests + differential fuzz) and micro-benchmark gates, but requires full human review. Do not merge without expert scrutiny.

What changed

Optimizes spark_unhex in the native spark-expr crate.

Replace unhex's per-digit range-match with a compile-time 256-entry hex lookup table and preallocate the BinaryBuilder to its known output size, cutting per-byte branching and repeated buffer reallocations.

Evidence

• Correctness: unit tests + seeded differential fuzz (bit-identical Arrow output vs main).
• Benchmark (criterion, Rust-only): see numbers below; gate required ≥ 5.0% mean improvement with non-overlapping confidence intervals and no regressions.

spark_unhex_ valid mixed hex: 19.964% faster (base 125189ns -> cand 100197ns)
spark_unhex_ mixed hex column: 13.986% faster (base 109637ns -> cand 94303ns)
spark_unhex_ all valid: 42.128% faster (base 201069ns -> cand 116362ns)
spark_unhex_ with nulls: 36.465% faster (base 179663ns -> cand 114150ns)
spark_unhex_ valid hex: 22.177% faster (base 152680ns -> cand 118821ns)