This work establishes the mathematical foundations for geometric arbitration and deterministic consensus through harmonic field geometry. We introduce geometric arbitration, a deterministic Byzantine consensus primitive based on rotor traversal over finite cyclic groups ℤₙ with projection-based geometric selection. Our core results prove that constant coprime strides produce complete traversal in exactly n steps, and establish bounded convergence guarantees. The framework is formalized axiomatically with rigorous proofs of safety (identical outcomes), liveness (finite bounded termination), and completeness (all valid configurations reachable). We prove Byzantine fault tolerance under the DLS partially synchronous model, tolerating f < n/3 faults with O(n²) message complexity. This provides the mathematical substrate for the SevenFold Proof of Consensus protocol, whose implementation and operational characteristics are described in companion publications on Zenodo.