Resolves the halo color for text rendering on the given node.
Returns transparent when no halo should be drawn (explicit transparent only).
Missing halo is treated as 'inherit'.
When value is 'inherit' (or missing), resolves: node fillStyle (if not transparent) →
diagram background → a fallback contrast color derived from the text color.
Any candidate that does not contrast sufficiently with the text color is skipped.
Resolves the halo color for text rendering on the given node. Returns transparent when no halo should be drawn (explicit transparent only). Missing halo is treated as 'inherit'. When value is 'inherit' (or missing), resolves: node fillStyle (if not transparent) → diagram background → a fallback contrast color derived from the text color. Any candidate that does not contrast sufficiently with the text color is skipped.