Geodesic Focusing on CP^n and the Quantum Computing Scaling Ceiling: Why Euclidean Calibration Fails Exponentially

We prove that Euclidean (flat-geometry) state estimation in quantum computing introduces systematic error that grows as 2(n+1) with the complex projective dimension n = 2^q - 1 of a q-qubit system. This growth is a direct consequence of the Raychaudhuri focusing equation on CP^n equipped with the Fubini-Study metric (holomorphic sectional curvature K=4). The focusing is inescapable: we prove that every geodesic congruence on CP^n reaches a caustic in finite time, regardless of initial expansion rate. At 10 qubits, the predicted Euclidean calibration error exceeds 1,700%. At 50 qubits, it exceeds 10^18%. We validate against real IBM quantum hardware data (ibm_fez, 156-qubit processor), reproducing the published Bures advantage to within 0.001%: Bell state +0.415%, GHZ state +1.332%. Independent simulation confirms Bures (Riemannian) averaging outperforms Euclidean averaging in 100% of trials across 1-5 qubits (50/50). The correction is a software modification requiring no hardware changes.