nx_uxf_mp4box_gate.nx

nx_uxf_mp4box_gate.nx -- the D001 MIGRATION of nx_uxf_mp4box_test onto nx_gate_verdict (2026-09-06). IT DRIVES THE EXTRACTED LIB, NOT A THIRD COPY. nx_uxf_mp4box_test carried its own mp4_w32, mp4_r32, mp4_4cc, mp4_box_begin and mp4_box_end, which are the same primitives now shipping in nx_uxf_remux.nx as uxr_w32, uxr_r32, uxr_4cc, uxr_box_begin and uxr_box_end. Pointing the teeth at the lib keeps ONE MP4 box writer in the estate. The 4cc comparison is done here with the lib's own uxr_bytes_eq rather than by adding a helper to a lib that is already bite-proven, because editing a proven lib to satisfy a test invalidates the proof for no capability gain. ALL FOUR ORIGINAL TEETH PRESERVED IN PREDICATE, split where the original bundled several claims into one boolean: the walk's ALIGNMENT and its BOX COUNT are separate teeth, and each of the three expected box types is its own tooth, so a failure names which box is wrong rather than reporting that the tree is wrong. license_tier: ORIGINAL No hw writes (Rule 26).