Function apply_selout 

pub fn apply_selout(
    grid: &[String],
    fallback: Option<&str>,
    mapping: Option<&HashMap<String, String>>,
) -> Vec<String>

Apply selective outline (sel-out) transform to a grid.

Selective outline varies the outline color based on adjacent fill pixels, creating softer edges. For each outline pixel (a pixel adjacent to both opaque and transparent pixels), the transform picks a color based on the most common neighboring fill color.

Arguments

• grid - The grid of token rows
• fallback - Optional fallback token for pixels where neighbor can’t be determined
• mapping - Optional explicit mapping from fill token to outline token. Key “*” serves as the default fallback.

Returns

A new grid with outline pixels recolored based on neighbors

Algorithm

1. Parse each row into tokens
2. For each pixel, check if it’s an “outline” pixel:
   • An outline pixel is opaque (not {}) and adjacent to at least one {}
3. For outline pixels, find the most common non-transparent neighbor
4. Map that neighbor to an outline color using the mapping or by appending “_dark” to the token name