feat: decode arrays into [N]T and add MarshalAppend

Assisted-by: GLM 5.3 Flash

interpres.go

@@ -399,6 +399,17 @@ func Marshal(v any) ([]byte, error) {
 	return MarshalContext(context.Background(), v)
 }
 
+// MarshalAppend appends the TOML encoding of v to buf and returns the extended
+// buffer, the shape json.MarshalAppend has. A failed encoding leaves buf
+// untouched and comes back with a nil slice.
+func MarshalAppend(buf []byte, v any) ([]byte, error) {
+	out, err := Marshal(v)
+	if err != nil {
+		return nil, err
+	}
+	return append(buf, out...), nil
+}
+
 // MarshalContext is the cancellable variant of Marshal.
 func MarshalContext(ctx context.Context, v any) ([]byte, error) {
 	if err := ctx.Err(); err != nil {