811: multicolor node support

[glstott]

This addresses GH enhancement issue #811

Summary

• Added a “Mixed Values” node color option that renders delimited node color values, such as 2a/3a, as multi-color node fills when coloring nodes by a field.
• Added shared mixed node color parsing and fill-style support:
  • splits supported delimiters into component color categories
  • preserves color/transparency mapping per component
  • treats empty, null, undefined, NaN, N/A, and n/a as missing values
• Updated node color tables to show positive fractional mixed counts instead of hiding values below 1.
• Added mixed-color rendering across 2D, Map, Bubble, and Phylogenetic views.
• Fixed mixed-color border rendering so simple multi-color nodes use the same visible border thickness as single-color nodes, while custom icon shapes that should not use borders still suppress them.
• Added SVG export handling so mixed-color node fills export as vector gradients where needed.
• Added mixed node shape SVG helpers and unit coverage for gradient fills, custom shape clipping, border scaling, and stroke suppression.
• Added Cypress mixed-genotype fixtures and a mixed node coloring journey covering 2D, Bubble, Map, and Phylogenetic views.