Add MVP ambient biome audio hook

Docs/TechnicalDesignDocument.md

@@ -302,15 +302,19 @@ the represented local day/night cycle and current weather without hard-coded
 static light settings. The Ground Zero map setup script places this controller
 and removes the earlier static demo sun/skylight/fog actors.
 
-First-pass weather audio uses `AAgrarianWeatherAudioController`. The controller
-owns ambient, rain, wind, and storm audio components with assignable loop sound
-slots. It reads replicated weather state, provider wind speed, provider cloud
-data, and local night/day state, then fades component volumes so rain, wind, and
-storm cues follow the same authoritative weather mapping used by temperature and
-lighting. The current MVP can ship without final sound assets because the
-controller is silent until loops are assigned; placeholder or final audio can be
-added by setting the exposed sound properties on the placed controller or a
-Blueprint child.
+First-pass biome and weather audio uses `AAgrarianWeatherAudioController`. The
+controller owns ambient, rain, wind, and storm audio components with assignable
+loop sound slots. The ambient component is explicitly the Ground Zero
+coastal-scrub biome bed through `BiomeAmbientLoopSound`, with separate day and
+night volume targets so the map can carry wind, surf, insects, distant birds, or
+other realistic low-level biome texture before final authored assets are
+available. It reads replicated weather state, provider wind speed, provider
+cloud data, and local night/day state, then fades component volumes so rain,
+wind, and storm cues follow the same authoritative weather mapping used by
+temperature and lighting. The current MVP can ship without final sound assets
+because the controller is silent until loops are assigned; placeholder or final
+audio can be added by setting the exposed sound properties on the placed
+controller or a Blueprint child.
 
 Campfires expose native extinguish logic through `AAgrarianCampfire::Extinguish`.
 Extinguishing clears remaining fuel, turns off replicated lit state, and reuses