[kumo] Make Button emphasis foreground themeable

[ftzi]

Issue

Primary and destructive Buttons in @cloudflare/kumo@2.12.0 force a white foreground even though their emphasis backgrounds come from semantic theme colors. When a custom theme uses a light brand or danger color, Button text can lose contrast, and the theme has no semantic foreground hook for choosing a dark color.

On current main, KUMO_BUTTON_VARIANTS applies !text-white to both primary and destructive. In the same component, getEmphasisStyle() derives the background, gradient, and ring from --color-kumo-brand or --color-kumo-danger, so those surfaces respond to theme values while the foreground remains fixed.

For example, a theme that sets a light --color-kumo-brand or --color-kumo-danger value renders a light emphasis surface while the computed foreground stays white. Ordinary text utilities and stylesheet rules lose to !text-white, so a dark foreground currently requires an !important override.

Solution

• Add the narrowly scoped semantic token text-kumo-button-emphasis, backed by --text-color-kumo-button-emphasis.
• Default the token to white in both light and dark modes, preserving existing Kumo visuals.
• Replace !text-white with !text-kumo-button-emphasis for primary and destructive variants. The Button declaration keeps its existing precedence, but now reads a theme variable that consumers can override without !important.
• Verify enabled, disabled, and loading states; disabled and loading opacity remain 0.5.
• Update generated theme CSS, semantic token documentation, AI usage guidance, downstream Banner expectations, and the package changeset; verify registry and Figma generation consume the new token.

Validation

• Static gates: formatting, lint, workspace typecheck, lockfile stability, and diff checks passed.
• Tests: 52 unit files with 1,278 tests, 2 focused Chromium tests, and 197 export-path assertions passed.
• Builds and generation: Kumo package and docs builds passed; registry and Figma generation consumed the new token.
• Release metadata: changeset status and Kumo changeset validation passed.

Remaining limits

• Computed-style browser coverage ran in Chromium only; no Safari or Firefox manual run was performed.
• This PR addresses only primary/destructive foreground theming. It does not resolve the broader secondary, outline, or hover concerns tracked in [docs] Button variants missing hover styles and using non-semantic color tokens #35.

AI disclosure

This change and pull request description were prepared by an AI coding agent at the contributor's direction.

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