nx_av1_intra_gate.nx

nx_av1_intra_gate.nx -- proves AV1/AV2 non-directional intra prediction. T2 pins DC's FOUR availability cases. With NEITHER edge the answer is mid-grey 1 << (bitdepth-1), not zero. A decoder that treats a missing edge as zeros predicts black along the top and left of every frame -- which reads as a vignette, not as a bug. T5 is the strongest structural check for the smooth family: with CONSTANT neighbours every smooth predictor must return exactly that constant. That holds only if each weight and its complement sum to 256 and the rounding matches. Using 255 as the complement produces almost-right constants -- a fraction of a level per pixel, accumulating across a frame as a gradient. T4 pins Paeth's TIE ORDER and that it SELECTS rather than averages: over 300 random triples the result must always be one of its three inputs. license_tier: ORIGINAL