nx_gamemusic_gate.nx

nx_gamemusic_gate.nx -- CERTIFICATION of nx_gamemusic, completing `audio-sfx-music`. The SFX half is already proven by nx_game_audio_gate (3/3, real WAV). This gate proves the MUSIC half so the capability can honestly be called HAVE rather than inferred from SFX alone. T1 EXACT equal-temperament octaves: A4=440 -> A5=880 -> A3=220, no drift T2 DETERMINISTIC render: same song rendered twice -> byte-identical samples T3 DATA-DRIVEN: a different song array produces different audio (melody is data, not code) T4 SATURATING MIX: layering loud tracks clamps instead of wrapping (no integer-overflow crackle) T5 PER-NOTE ENVELOPE: a note attacks then decays (it does not click on at full amplitude) T6 ★ANTI-VACUITY: output is non-silent AND pitch is real -- a high note has strictly more zero-crossings than a low note. Silence or a constant would satisfy T2/T4 vacuously. T7 REAL ARTIFACT: a playable WAV is written to disk via the existing nx_audio_wav writer T8 composes with nx_gamesave: the song DATA round-trips through save/load license_tier: ORIGINAL expect_exit: 0