bench: size the decimal kernel benchmarks by bytes rather than rows - #9980

Summary

The decimal kernel benchmarks landed with a 1,024 row case. For i128 that runs in 0.4 to 1.2 µs
on the walltime legs, below what those machines resolve, so it reports mostly jitter: it moved by
10 to 19% on ten of the last thirteen CodSpeed reports, every one of them on a pull request that
touched no decimal code. No 8,192 row case and no i256 case has moved in those reports.

Changes

Derive each kernel case's row count from a working set of 256 KiB and 1 MiB of input plus output,
rather than fixing it at 1,024 and 8,192 rows:

Both widths now do comparable work, every case takes several microseconds instead of fractions of
one, and the largest still fits the 1 MiB L2 cache of the Graviton leg, so these stay measurements
of kernel code rather than of memory bandwidth.

The array benchmarks in the same files are #[cfg(not(codspeed))] and never run in CI, so they
keep their sizes for quick local runs. The benchmarking guide gains the measured walltime floor
next to the simulation floor documented in #9951.

Expect a one-time batch of CodSpeed rows for these eight cases when this merges, since the inputs
change size.

Checked with cargo clippy -p vortex-decimal-byte-parts --all-targets --all-features -- -D warnings
and cargo fmt. No benchmarks were run locally: this session is a shared cloud VM, so its walltime
numbers would not mean anything.

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