Fix color contrast failures in default and colorblind/tritanopia themes

[sneharao3]

Summary

Refs github/core-ux#2821

Fixes failing color-contrast checks across the default light/dark themes (now fully passing) and reduces failures in the colorblind/tritanopia themes. All changes are accessibility-only token repoints — no hue changes or brand-affecting shifts. Targets merge-34-56-test, since the fixes depend on its neutral-scale refactor.

List of notable changes

• updated fgColor.onEmphasis (dark) to white — was a light gray on emphasis backgrounds, failing ~3.2–4.5:1
• updated fgColor.muted / fgColor.neutral to neutral.9 (the text step) — were on neutral.7, a background step; placeholder, closed, and invisible-icon tokens inherit this
• updated control.borderColor.emphasis (default themes) to the softest neutral step clearing the 3:1 border threshold
• updated button.danger and button.primary text tokens to white to match onEmphasis
• updated bgColor.neutral.emphasis (light) so white text passes

Contrast check results

Failing pairs per theme, before → after:

Theme	Before	After
light	15	0
dark	44	0
light_protanopia_deuteranopia	21	2
dark_protanopia_deuteranopia	48	2
light_tritanopia	21	2
dark_tritanopia	48	2
light_high_contrast	16	15
dark_high_contrast	37	10
light_protanopia_deuteranopia_high_contrast	21	19
dark_protanopia_deuteranopia_high_contrast	39	15
light_tritanopia_high_contrast	21	19
dark_tritanopia_high_contrast	39	15
dark_dimmed_high_contrast	11	11

What should reviewers focus on?

• The step choices on the gray tokens — neutral.9 is the lightest/darkest step that clears 4.5:1 and is the designated text step.
• Two known gaps, intentionally left for follow-up:
  • The colorblind/tritanopia themes still have 2 failing control-borderColor-emphasis rows each — those variants have their own border override the default-scoped fix didn't touch.
  • The high-contrast themes are not addressed here so the HC contrast checks will still show failures in CI by design.

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[lukasoppermann]

@sneharao3 why did you delete the colors from the story here?

[sneharao3]

I think it's showing because this branch is based on merge-34-56-test and is picking up some of those commits. I don't think this one is from my changes since my changes are only the token repoints in the color JSON files. How do you think I should handle this?

[lukasoppermann]

@sneharao3 why is this not using fgColor.white, does that make sense?

[sneharao3]

Good catch! You're right, this should be fgColor.white. That was an unintended change