feat(mindmap): add coloring by hierarchy depth

[jackroberts236]

Summary

Adds a mindmap.colorBy option so mindmaps can be colored by hierarchy level instead of by branch.

• branch (default): current behavior, so existing diagrams are unchanged.
• depth: nodes at the same distance from the root share a color across all branches.

---
config:
  mindmap:
    colorBy: depth
---
mindmap
  Root
    Planning
      Tasks
    Delivery
      Releases

Loading

The same mindmap with branch (left) and depth (right), in the classic look with the default theme and the neo look with redux-color:

This PR also fixes HTML labels in the neo look. SVG text switches color with the backgrounds a theme paints, but HTML labels kept the classic label colors. The root label was unreadable on dark and forest (1.2:1 and 1.4:1 contrast), and so were the labels of the second section on neutral (#F4F4F4 text on #eee). This affected branch coloring too, and depth coloring made it more visible. Section labels in redux-dark also change to match the neo text color, so its existing neo snapshot will differ slightly.

Resolves #2

Design decisions

• Depth mode only changes which section each node is assigned to. The root keeps its own color, depths 1–11 use cScale1–cScale11, and deeper levels repeat. Styling reuses the existing section rules, so every theme and look treats depth colors the same way it treats branch colors, with no new theme variables.
• Depth counts parent-child relationships, not indentation, so uneven indentation and indented roots color correctly.
• Neo HTML labels (span) now use the same color as the neo SVG text rules, for section nodes and the root. A Playwright test checks label contrast on the repainted themes in both modes. As a result, the neo HTML root label now uses cScaleLabel0 instead of gitBranchLabel0. Palettes that set only gitBranchLabel0 should also set cScaleLabel0. With useGradient: false, the dark root label drops to 2.8:1, the same contrast its SVG text already had. I kept the SVG rule rather than picking the variable by background, because that would drop forest with useGradient: false from 10.7:1 to 2:1.
• Open PR fix: preserve mindmap label colors in exports mermaid-js/mermaid#8263 also edits mindmap label styles: it adds a color to .mindmap-node-label. The current renderer doesn't emit that class, and this PR's rules target the label span directly, so the two don't interact and can merge in either order.

[jackroberts236]

Moved upstream: mermaid-js#8394