77c2d7bce9
Throwaway diagnostic for the multi-core-vs-JIT decision. Two real numbers: 1. Existing --parallel is ~25x SLOWER, not faster: 100.9s wall vs 4.0s lockstep at -n 200M --gpu-inline (237s user across 6 threads burned on coarse-mutex + phaser contention). Naive coarse-locking inverts the win. 2. XENIA_CONCURRENCY_PROBE (env-gated, zero cost off): per-round histogram of runnable HW-slot width. Sylpheed at -n 300M: AVG WIDTH = 3.83 (width>=4 in 77% of rounds) => a *perfect* host-thread-per-guest-thread design has a ~3.8x Amdahl ceiling. The parallelism is real; the existing vehicle just can't capture it. Golden n200m byte-identical (probe inert unless env set). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>