Add per-edge TMAP tree colors

[lhallee]

Summary

• Add TmapViz.set_edge_colors(colors) for one hex color per tree edge.
• Serialize per-edge RGB payloads in inline HTML and static output.
• Render per-edge colors in the official WebGL edge layer while preserving the existing global edge style path.

Validation

• Added visualization tests for serialization, length validation, and backwards-compatible global edge styling.
• Syntax-only parsing was run on the workstation; pytest was intentionally not run yet.