fix: use CPU-aware prover scheduler

[jotabulacios]

This PR fixes the CPU proving regression introduced by the table scheduler.

The GPU scheduler uses external driver threads to admit and overlap table work. On CPU, that caused table tasks to launch nested Rayon work from outside the Rayon pool, leading to oversubscription, cache contention, and memory-bandwidth contention.

The CPU path now:

• schedules table work through the existing Rayon pool;
• limits table concurrency in bounded chunks; and
• separates auxiliary-trace construction/commitment from rounds 2–4 with a CPU phase barrier.

The CUDA path and proof format are unchanged.

[jotabulacios]

/bench

[github-actions]

Benchmark — real block (ethrex_mainnet_25368371.bin) (median of 3)

_{continuations · epoch 2^22 · 13 epochs}

Metric	main	PR	Δ
Peak heap	48519 MB	53151 MB	+4632 MB (+9.5%) 🔴
Prove time	166.381s	146.118s	-20.263s (-12.2%) 🟢

🎉 Improvement on the real block — prove time down 12.2%.

_{Prove-time spread 1.6% (147.254s / 146.118s / 144.936s)}

_{Commit: 62301c8 · Baseline: cached · Runner: self-hosted bench}