Bound RMS release proof concurrency

[MauScheff]

Summary

• serialize the native RMS test harness in the release job
• limit nested Cargo compilation to two jobs
• prevent hosted-runner starvation while preserving the full proof suite

Evidence

• two hosted runners lost communication during the previous default-parallel native proof step
• both failures had no Rust assertion failure and occurred before strict audit
• exact constrained local command passed: 637 unit tests, 7 CLI tests; one external CVC5 test ignored and the provider test handled separately
• rms check --committed --root . passed on the workflow-fix commit