273: continuous color ramps

[glstott]

This addresses #273, adding support for continuous color palettes in node and link colors.

Summary

• Add continuous color-ramp support to node and link color assignment files.
• Allow CSV and TSV assignment files to specify either categorical or continuous mode.
• Validate continuous assignments to ensure:
  • Ramp values are finite numbers.
  • At least two distinct color stops are provided.
  • A file does not mix assignment modes.
• Preserve independent custom color assignments for nodes and links.
• Apply imported continuous domains and colors across supported visualizations.
• Add node and link assignment controls to Global Settings.
• Add descriptive continuous legend labels, such as:
  • Node Color: Degree
  • Link Color: Distance
• Include continuous legend labels and accessible titles in SVG exports.
• Position exported continuous legends beside the visualization so they do not cover the view.
• Prevent continuous legends and endpoint labels from being clipped in PNG exports.
• Add documented example node and link continuous-ramp assignment files.