Why the Strong CP Problem Is Solved: thetaQCD = 0 from F₂1 Group Theory

The strong CP problem is one of the deepest mysteries in particle physics: the QCD Lagrangian contains a term proportional to thetaQCD that would generically cause the strong force to violate CP symmetry (the combined symmetry of charge conjugation and parity). But experiments show thetaQCD is smaller than 10^-10 — essentially zero — with no known reason why. This tutorial explains why the strong CP problem is automatically solved in the GTE framework. The F₂1 symmetry group (the full symmetry group of the GTE polynomial over Z₇, of order 42) contains a reflection element that maps p to its CP-conjugate; the vanishing of thetaQCD follows from this structural symmetry as a theorem, not a coincidence. The derivation is machine-certified in Lean 4 with zero sorry.