Adjust Prism xychart series palette

[jongalloway]

Closes #143.

Summary

• switch Prism-style xycharts away from the accent-only ladder when the theme has a monochrome NodePalette and multi-stop BorderGradientStops
• derive xychart series colors from ThemePaletteResolver.ResolveEffectivePalette(...) and soften them toward the background so bars stay readable without taking on the old vivid solid-blue look
• keep existing accent-driven xychart colors for non-Prism themes
• add a focused renderer test for Prism xychart series colors
• add a Prism xychart E2E fixture and snapshot baseline

Validation

• dotnet test tests/DiagramForge.Tests/DiagramForge.Tests.csproj -c Release --filter "FullyQualifiedName~Render_XyChart_WithPrism_UsesSoftenedBorderGradientSeriesColors"
• dotnet test tests/DiagramForge.E2ETests/DiagramForge.E2ETests.csproj -c Release

Notes

This implements the middle-ground approach from the issue: Prism bars use lighter, softened series colors derived from the theme's border-gradient palette rather than staying fully white or using the previous accent-only ladder.

[Copilot]

Pull request overview

Updates Mermaid xychart-beta series fill colors to better match the Prism theme’s “border-carries-emphasis” style by deriving series colors from the theme’s effective palette (gradient-stop-derived for Prism) and blending them toward the background for readability, while leaving non-Prism themes on the existing accent-driven ladder.

Changes:

• Adjust SvgRenderSupport.GetXyChartSeriesColor() to use ThemePaletteResolver.ResolveEffectivePalette(...) for Prism-like themes (monochrome NodePalette + multi-stop BorderGradientStops), then soften fills via blending.
• Add a focused unit test asserting Prism xychart series colors come from the softened effective palette (and not the prior accent ladder).
• Add a Prism xychart E2E fixture + snapshot baseline to lock in the rendered SVG output.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
src/DiagramForge/Rendering/SvgRenderSupport.cs	Switches Prism-like xychart series fills to a softened effective-palette strategy; keeps legacy accent ladder otherwise.
tests/DiagramForge.Tests/DiagramRendererTests.cs	Adds a renderer test validating Prism xychart series colors use softened effective palette colors rather than accent-ladder colors.
tests/DiagramForge.E2ETests/Fixtures/mermaid-xychart-prism.input	New Prism-themed Mermaid xychart fixture input for snapshot coverage.
tests/DiagramForge.E2ETests/Fixtures/mermaid-xychart-prism.expected.svg	New snapshot baseline capturing the expected Prism xychart rendering with the updated series fill colors.