nx_av1_frame_gate.nx

nx_av1_frame_gate.nx -- proves the AV1/AV2 frame-header parameter blocks. T1 pins su() at its ACTUAL widths. A delta-Q is su(1+6) -- seven bits, not eight. Sign-extending from the wrong bit turns -1 into +63 and every negative delta into a large positive one; the frame then decodes at the wrong quantiser with no error anywhere. The test walks every value of a 7-bit signed field, so an off-by-one in the sign mask cannot survive. T4 pins the CDEF secondary-strength gap: coded 0,1,2,3 means strength 0,1,2,FOUR. Strength 3 does not exist. A writer storing 4 verbatim overflows a 2-bit field; one storing 3 silently means 4. The gate asserts the mapping in both directions AND that strength 3 is REFUSED as unrepresentable rather than quietly coerced. license_tier: ORIGINAL