add AssetFS hook so assets can load from an embed.FS (js/wasm)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

asset.fs.go

@@ -0,0 +1,31 @@
+package pncdsl
+
+import (
+	"io/fs"
+	"os"
+)
+
+// AssetFS, when non-nil, is the filesystem asset files (images, audio)
+// are read from instead of the OS filesystem. Set it to an embed.FS
+// before Run to ship assets inside the binary — required for js/wasm
+// builds, where there is no OS filesystem and every asset would fall
+// back to a placeholder:
+//
+//	//go:embed assets
+//	var assetsFS embed.FS
+//
+//	func init() { pncdsl.AssetFS = assetsFS }
+//
+// Asset.Path values are used as-is, so they must be slash-separated
+// and relative (e.g. "assets/bg/bedroom.png") — the same form that
+// works with os.Open from the project root.
+var AssetFS fs.FS
+
+// openAsset opens an asset file honoring AssetFS. Every asset read in
+// the package goes through here.
+func openAsset(path string) (fs.File, error) {
+	if AssetFS != nil {
+		return AssetFS.Open(path)
+	}
+	return os.Open(path)
+}