nx_av1_seq_gate.nx

nx_av1_seq_gate.nx -- proves the AV1/AV2 sequence header by ROUND-TRIP. T3 is the one worth having: the frame dimension is written in a field whose WIDTH is itself coded immediately before it. A parser using a fixed width happens to work near 16-bit sizes and mis-parses everything after the field on any other size, because the bit cursor is then wrong for the whole rest of the header. T3 round-trips dimensions spanning 1 to 65536 -- 1x1, 64x64, 1920x1080, 3840x2160, 65536x65536 -- so a fixed-width reader cannot pass. T5 covers the AVIF shapes specifically: 8/10/12-bit, monochrome, and 4:2:0 vs 4:4:4, since those are what a still image actually carries. NON-VACUITY: T6 covers refusals -- a non-reduced header (unsupported here, and refused rather than half-parsed), a bad profile, 12-bit outside profile 2, and zero dimensions. license_tier: ORIGINAL