How to write a Venus negative control (falsifier) that actually discriminates, explicit atomic setter lever called from main(), one printed sentinel line, PASS_REGULAR_EXPRESSION-only ctest registration (never bare WILL_FAIL), the CTest inversion semantics, and the MSVC env-lever trap. Use when adding a WILL_FAIL twin, when a control passes vacuously, or when a reviewer says "not discriminating".
