The spec-first approach you describe crystallizes a fundamental shift: when LLM-generated code becomes the implementation layer, debugging moves from "inspect the artifact" to "negotiate with the specification language." I've observed this in production systems where teams end up maintaining three parallel sources of truth—the spec, the test corpus, and a cached reference generation—because regeneration non-determinism breaks deployment confidence. The "library with no code" experiment exposes what happens when that brittleness becomes the design rather than a side effect. The insight about tests moving upstream is especially sharp: they're no longer validators but constraints on a probabilistic search space.