Document MVP weather sound hooks

Docs/Audio/WeatherSounds.md

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+# MVP Weather Sounds
+
+Weather sounds are owned by `AAgrarianWeatherAudioController`, which is placed
+in the Ground Zero demo map as `AGR_DemoWeatherAudioController`.
+
+The controller exposes these sound slots:
+
+- `RainLoopSound`
+- `WindLoopSound`
+- `StormLoopSound`
+- `ClearAmbientSound`
+- `BiomeAmbientLoopSound`
+
+The MVP weather audio path is intentionally asset-agnostic. The packaged build
+remains silent until placeholder or final loops are assigned, but the runtime
+logic already follows server-replicated weather state, provider wind speed, and
+day/night state. Rain, wind, and storm volumes fade independently so weather can
+move from clear coastal ambience to rain, cold wind, or storm without changing
+gameplay code.
+
+Final assets should remain grounded and local to Ground Zero: coastal wind,
+distant surf, light rain on brush, heavier storm gusts, and subtle night/day
+texture. Weather audio must support the non-ray-traced default investor build
+and headless multiplayer server builds by keeping actual playback client-side.