D-068 bus layout (Music, Ambient, WorldSFX, PlayerActions, UISounds), directory-scan asset registry, spatial/non-spatial playback, D-069 audio dip profiles (dialogue, confrontation, listening_focus) with low-pass filter sweep, and D-073 zone crossfade stub. No-op fallback when audio assets absent. Implements #255. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>