nx_roundtrip_gate.nx

nx_roundtrip_gate.nx -- THE GATE FOR THE SELF-ORACLE. SUBJECT: the nx_roundtrip ELF, forked for real. Its contract is a FOUR-STATE EXIT (0 IDENTICAL / 1 LOSSY / 2 usage / 3 UNOBSERVABLE) and /api/gate_run derives its verdict from an exit code, so every tooth here asserts an EXIT CODE. A gate whose exit does not carry its verdict silently blesses every failure it finds. THE FIXTURES ARE MIXED ON PURPOSE, AND THE REASON IS MEASURED. The two POSITIVE fixtures are real pipeline artifacts (nx_gltf2mesh's own conversion of two donor GLBs), copied to a durable knowledge/ path and byte-verified. Three hand-authored containers were tried first and every one went RED for a reason unrelated to fidelity -- metre-scale coordinates were rescaled on the return leg, millimetre ones were refused outright, unit normals changed nothing -- because a synthetic container cannot reliably reproduce the canonical regime the pipeline emits, and a fixture the subject rejects tests the fixture rather than the subject. The NEGATIVE fixture stays synthetic and IS built at runtime, because a container truncated BY CONSTRUCTION is the one thing no real artifact can supply. THE BITE PAIR IS T1/T2 AND IT IS THE POINT. A checker that returned IDENTICAL for everything would pass any single positive test; a checker that returned LOSSY for everything would pass any single negative test. Only the PAIR discriminates. The two fixtures differ in exactly one property -- layer count -- so the pair isolates the one axis under test. THE FIXTURES' SHAPE IS ASSERTED BEFORE ANY OUTCOME IS. The positive fixtures are read, not written, so their presence AND their layer counts (1 and 9) are proven at setup: a missing fixture, or one that is not actually multi-layer, would make T1/T3 report on something other than the property they name. A NAMED absence, never a silent skip. license_tier: ORIGINAL No hw writes (Rule 26). expect_exit: 0