Hi Ragnar
It was made manually, though it could be covered by by an algorith more or less I guess.
The idea came from the way I would ink such shapes with a pen on paper.
There, staying between the outlines is as important to me as the good coverage.
And, drawing parallel lines is much safer than fast turns next to the borders.
Automating the method a bit: by insetting/dynamic offsetting of the path, you can draw a "center-line" of the shape.
As those are parallel, there will be islands at the end, that need to be connected. This line and the original path's interpolation would produce a good source to the fill theoretically.
With the heart-shaped spiral the distance between those were sorted evenly, in this example above I tried to connect them with a small turn.
If it was about the consistent fill, then maybe a straight hatching would be the best, perpendicular to that centerline -as used in embroidery and graffiti.