Bevy's HttpWasmAssetReader fetches asset paths over HTTP at runtime, requesting 'assets/shaders/*.wgsl' (matching AssetPlugin's default file_path of "assets"). Trunk only emits what index.html references, so without an explicit copy-dir link the shaders were absent from dist/ and every material hit a 404 → grey screen. Add the copy-dir link so trunk mirrors assets/ into dist/.