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/.
