Quantum Spacetime Erasure: pi1 != 0 and [x,y]=1 Cannot Both Vanish Without an Eraser

We prove one exact theorem in a finite toy model and give one falsifier for a two-path interferometer. Let A1 = k<x,y>/(xy-yx-1) be the Weyl algebra, C*(S^1;k) the cochains on a circle, T = A1 tensor C*(S^1;k), and E = k the empty history. Then pi1(T)=k != 0 and [x,y]=1 != 0 in T, while pi1(E)=0 and [x,y]_E=0. No unital algebra map T -> E exists. The only way to kill both witnesses is the eraser x,y -> 0, C^1 -> 0, which is a linear projection, not an algebra map. We translate this to Englert-Greenberger duality: D measures pi1, V measures [x,y]. The model predicts D ~ 1 and V ~ 1 cannot co-occur in the same run without an eraser. A repeatable >3 sigma violation with no eraser falsifies the model. We do not claim proof of chronology protection in full general relativity. We claim the toy theorem is exact and the experimental test uses existing data. Standalone version of Chapter 124. Future work: H^n triple (Ch. 125), char p Frobenius (Ch. 126), categorical H (Ch. 127).