Fix zero-valued token colors

[koriyoshi2041]

Summary

• return the documented neutral white color for exact zero values
• add a regression for an all-zero token-color domain

Why

When value, min, and max are all zero, the positive branch evaluates 0 / max as 0 / 0. The resulting NaN mix is materialized as transparent black, so the foreground selector chooses white text and makes zero-valued tokens unreadable on the white page background.

Verification

• Node 18 Jest: 6 suites, 27 tests passed
• TypeScript typecheck
• ESLint
• browser and Node builds
• git diff --check

Closes #92.