Why the Strong CP Problem Is Solved: theta_QCD = 0 from F_21 Group Theory

The strong CP problem is one of the deepest mysteries in particle physics: the QCD Lagrangian contains a term proportional to theta_QCD that would generically cause the strong force to violate CP symmetry (the combined symmetry of charge conjugation and parity). But experiments show theta_QCD 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 F_21 symmetry group (the full symmetry group of the GTE polynomial over Z_7, of order 42) contains a reflection element that maps p to its CP-conjugate; the vanishing of theta_QCD follows from this structural symmetry as a theorem, not a coincidence. The derivation is machine-certified in Lean 4 with zero sorry.