fix(a11y): raise dark primary and body text contrast tokens (A7, A8)

[BrianGenisio]

Summary

Closes #11 ([a11y][A7]) and #12 ([a11y][A8]). Shared semantic tokens now meet WCAG AA contrast against Backgrounds-Main-Top in both themes. Visual change for all DS consumers — please confirm with design-system owners before merge.

Token changes

Token	Light	Dark
--Colors-Primary-Default (A7)	unchanged Primary-700 (5.05:1)	Primary-700 → Primary-450 (~5.5:1 vs #1D2740)
--Colors-Text-Body-Lighter (A8)	Neutral-800 → Neutral-950 (~5.7:1)	Neutral-900 → Neutral-700 (~5.6:1)
--Colors-Text-Body-Light	Neutral-900 → Neutral-1000 (keeps Light ≥ Lighter; fixes placeholders)	Neutral-750 → Neutral-650
--Colors-Buttons-Secondary-* (dark)	—	Default/Text → Primary-450; Hover → Primary-400; Pressed → Primary-500

Primary filled buttons still use --Colors-Buttons-Primary-* (Primary-700) and are unchanged.

Notes for consumers

• Do not fade --Colors-Text-Body-Lighter with opacity. Documented in colors/README.md. ChatCPT’s .composer__hint { opacity: 0.8 } will still fail in light until that opacity is removed (handle in the submodule bump PR).
• No learn_cosmo-chat submodule bump in this PR.

Test plan

• npm test — tests/contrast-tokens.spec.js measures Primary-Default ≥ 3:1 and body/secondary text ≥ 4.5:1 in light + dark; existing dropdown/slider suites green
• Design-system owner visual review (shared token impact)
• After submodule bump: re-measure ChatCPT sidebar labels, placeholders, secondary button; drop composer hint opacity if still failing

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📥 Commits

Reviewing files that changed from the base of the PR and between 4488dc4 and 1c8d8c1.

📒 Files selected for processing (5)

• colors/README.md
• colors/colors.css
• components/button/button.css
• tests/contrast-tokens.spec.js
• tests/fixtures/contrast-tokens.html

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