Neutral Cross-Framework Benchmarking Reveals Rz Sign Convention Divergence in Quantum Simulation Backends

We present CleitonForge, a neutral benchmarking layer for quantum circuit simulation implemented in Rust. By routing identical circuits through a canonical intermediate representation to four independent simulation backends (native statevector, quantrs2, roqoqo, q1tsim), we identify a sign convention disagreement in the Rz rotation gate that produces zero cross-backend fidelity on parameterized quantum circuits. The divergence is invisible to standard Clifford benchmarks and the IBM Quantum Volume test but produces exactly F=0 on QAOA and VQE circuits at specific rotation angles. We characterize the affected gate family, implement a convention normalization transpiler that restores fidelity to unity, and discuss implications for variational quantum algorithm portability across simulation frameworks. CleitonForge is open source (Apache 2.0) at https://github.com/cleitonaugusto/cleitonforge.