Feature Request: manual color additions with locks

While this somewhat flies in the face of the core premise of this tool, I'd like to see if one could define a max dark, max light, and middle color, lock them and then have the tool generate colors between those locked colors that would work with your method. I have some colors that I need to keep the middle tone aligned to a brand color but want to have light and dark steps that are equal in luminance and saturation across the steps.

No file or test entry point is named; begin by tracing Leonardo's existing color-generation flow and how it represents contrast, luminance, and saturation. Done means supporting locked dark, light, and middle colors while generating the intervening steps with the requested luminance and saturation behavior, with coverage for those cases.

Written by the indexing model from the issue text.