Prefetch nearby scene assets in the background

docs/grid-agent-operations.md

@@ -55,7 +55,11 @@ deployments should use absolute paths.
 
 Downloaded grid assets use the platform cache directory: `$XDG_CACHE_HOME/metacrate`
 or `$HOME/.cache/metacrate` on Linux, `$HOME/Library/Caches/metacrate` on macOS,
-and `%LOCALAPPDATA%\metacrate\cache` on Windows.
+and `%LOCALAPPDATA%\metacrate\cache` on Windows. The cache treats UUID-addressed assets as
+immutable, defaults to a 2 GiB LRU limit, and is configurable with
+`vision.asset_cache_max_bytes`. The live scene source continuously prefetches and prepares assets
+inside the configured view radius while the avatar is logged in; prefetching is stopped before
+logout and shutdown.
 
 ## Endpoint, grid identity, and authority