[Audio] Audio mixer with snapshots: combat, dialogue, exploration

[sja-thedude]

Goal\nGet the right thing loud at the right time.\n\n## Tasks\n- Snapshots per gameplay context\n- Crossfade durations tuned\n- Settings panel respects mixer

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Part of the NEXUS: The Sentient Galaxy initiative under PlayPath. Project lives at /projects/nexus-sentient-galaxy/.