refactor(bench): coherent benchmark matrix across all implementations

All implementations (inline, copy, rtrb, crossbeam) now test identical
workload shapes: single, burst_100, burst_1000 × u64, msg48, msg64.
Batch benchmarks remain mantis-only (copy ring). Removed non-comparable
[u8;64] and [u8;256] benchmarks. Split functions to satisfy clippy
too_many_lines lint.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Milerias