feat: Move StartupJS UI theme system to CSSX

[cray0000]

Summary

• migrate StartupJS UI to the CSSX-first theme model backed by CSS variables, component tag selectors, and :part() overrides
• compose UiProvider from CSSX Tailwind tokens, CSSX shadcn semantic tokens, and startupjsUiTheme.cssx.css, with app-provided StartupjsProvider style layered after UI defaults
• replace the old palette/Colors/CssVariables theme runtime with normal CSSX :root variables and semantic --color-* tokens
• remove public useThemeColor(), getThemeColor(), getThemeColorVariableName(), Colors, Palette, CssVariables, generateColors, and useColors APIs
• keep @startupjs-ui/core as a tiny non-style package for the UIRole type only
• refactor component JS color reads to CSSX useCssColor() / useCssVariable() primitives re-exported from startupjs
• inline all component, demo, MDX renderer, and docs styles into local css templates; only the package-level startupjsUiTheme.cssx.css theme asset remains
• replace Stylus helpers, $UI, u, :export config, and separate .cssx.styl files with CSS variables, rem, color-mix(), oklch(), and CSSX custom media
• preserve component customization through themed(name, Component), component tag selectors, semantic part props, and per-instance style / *Style props
• update documentation to present StartupjsProvider style, CSSX tokens, tag overrides, and :part() as the customization model
• keep the generic Styling guide as the canonical theming doc while component pages document only their own parts and variables/defaults
• memoize the UI provider style layer array so provider context does not churn when style is unchanged
• improve generated part reference extraction for conditional part arrays such as TextInput icon parts

Why

CSSX now owns provider-scoped :root variables, global utility classes, component tag selectors, :part() / ::part() overrides, subscribed CSS variable reads, OKLCH evaluation, and color-mix(). Keeping palette generation, theme-color hooks, theme context, component override wiring, or style helpers inside startupjs-ui would duplicate that model and keep theming in the wrong layer.

With this change, the StartupJS UI theme is normal CSSX CSS:

:root {
  --color-primary: oklch(0.55 0.22 260);
  --Button-radius: var(--radius-md);
}

Button {
  border-radius: 12px;
}

Button:part(text) {
  color: var(--color-primary-foreground);
}

Component color props still accept tokens such as primary, which map to --color-primary. Raw CSS values and var(--x) remain valid where component props accept raw colors.

Related PRs

• feat: Unify CSS-to-RN compiler and runtime pipeline cssx#5 adds the unified CSSX compiler/runtime, CssxProvider, provider-scoped :root, component tag selectors, CSS variable/color hooks, OKLCH/color-mix evaluation, custom media, runtime CSS support, and source-condition types.
• feat: Wire StartupjsProvider to CSSX startupjs#1327 wires StartupjsProvider style and theme into CSSX so framework users can configure this without importing CSSX directly.

Validation

• node scripts/update-style-reference-docs.mjs idempotency check
• yarn generate-package-dts
• node scripts/check-startupjs-ui-exports.mjs
• yarn build for the docs app
• git diff --check
• CSSX docs/provider smoke with local source links:
  • generic /docs/Styling theming guide renders and remains the canonical styling model doc
  • /docs/TextInput renders and includes conditional inputIconLeft / inputIconRight parts
  • component docs for Button, Item, Div, and DateTimePicker render their generated Styling reference sections
  • component reference sections list parts and component-specific variables/defaults and link back to the generic Styling guide
  • no unknown CSS color token warnings or page errors observed
• Storybook runtime smoke through http://localhost:4400/iframe.html:
  • Introduction/Overview
  • Actions/Button
  • Data/Item
  • Display/Badge
  • Feedback/Modal
  • Overlay/Popover
  • System/StartupJS UI
  • Inputs/TextInput
  • Inputs/DateTimePicker
• Visual screenshots checked for Styling, Button, Item, DateTimePicker, provider, and popover pages/stories.
• Known during DateTimePicker smoke: Moment Timezone logs its existing missing timezone-data warning; no CSS token, page, or provider errors were observed.
• Previous validation in this draft also included yarn install --immutable --mode=skip-build, targeted eslint over migrated packages/docs slices, and direct CSS-template compile smoke over packages/ plus startupjsUiTheme.cssx.css.

Review Focus

• token naming and override surface in startupjsUiTheme.cssx.css
• remaining public API compatibility that should intentionally stay or be removed for the breaking release
• part coverage on roots and semantically useful sub-elements
• JS-only color bridge usage where CSS variables could replace code
• docs clarity around StartupjsProvider style, CSSX variables, component tag selectors, and :part() overrides