FRI Soundness Above the Johnson Bound via Threshold Halving

We prove the first unconditional soundness theorem above the Johnson bound for FRI, STIR, and WHIR — the proximity-testing protocols underlying every deployed STARK, zkVM, and FRI-based system on Ethereum's roadmap. For $\mathrm{RS}[F, L, k]$ with $k = 2^m$ and $L$ admitting a fixed-point-free involution (standard for deployed FRI, in either characteristic), for every $\delta \in (\delta_J,\, 1-\rho)$: $$\varepsilon_{\mathrm{FRI}} \;\leq\; \frac{nR}{|F|} \;+\; \left(1 - \frac{\delta}{2}\right)^{