[C++] Optimize Decimal128 abs hotspot; add exec‑only TPC‑H Q1 benchmark

### Describe the enhancement requested

While profiling `BM_Tpch_Q1_ExecOnly in Acero`, a significant CPU hotspot appears in `BasicDecimal128::Abs()` (via decimal math in Q1). This PR proposes a small optimization to avoid temporary creation/operator< in `Abs()` and adds an exec‑only TPC‑H Q1 benchmark that isolates execution from data generation to make profiling/benchmarking more representative.

This enhancement should improve performance in decimal‑heavy compute and make it easier to identify execution hotspots in Acero.

### Component(s)

C++