Given P = R⁵ with sig(3,2), S₁ sig(3,1), S₂ sig(2,2), proves the decomposition is forced. Five steps: (1) dim(S₁ ∩ S₂) = 3, (2) W nondegenerate via Gram determinant, (3) sig(W) = (2,1) by intersection of embedding constraints, (4) orthogonal complements V₁ = S₁ ∩ W⊥ positive, V₂ = S₂ ∩ W⊥ negative, (5) adapted basis with off-diagonal entry α = ⟨V₁,V₂⟩ determined by fixed S₁, S₂. The downstream chain (Proof 21) depends only on W, not on α. Algebraic, no computation. Dependencies: Proofs 3, 4, 5, 16.
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