yeah went back and forth on exactly this trade-off, you're right that vertical can produce tests tailored to the impl. horizontal forces invariants up front but

yeah went back and forth on exactly this trade-off, you're right that vertical can produce tests tailored to the impl. horizontal forces invariants up front but the failure mode flips: you're tailoring tests to the architecture you imagined before any feedback from working code. so it's invariants-vs-behaviors, both have a tailoring failure mode just on different axes. compromise i landed on: vertical + an explicit anti-tailoring grill check at each cycle. definitely gonna tweak with more as i keep refining.