PoC: widgetAccentable prop + light-dark() color support (Track 1)

[burczu]

Closes #165

Summary

Standalone rendering improvements for Voltra widgets on iOS. Adds two building blocks that enable
native SwiftUI environment adaptation without touching the server or the AppIntent pipeline:

• widgetAccentable prop — a new boolean prop on all Voltra components, serialised as the
  short name wa. Applied via .widgetAccentable() in VoltraElementView so elements receive
  the system accent tint in accented / Liquid Glass rendering modes automatically.

• light-dark() color support — CSS Color Level 4 light-dark(<light>, <dark>) syntax
  accepted anywhere a color string is used. Parsed in JSColorParser and stored as a
  (light: Color, dark: Color) pair; rendered via LightDarkForeground: ShapeStyle whose
  resolve(in: EnvironmentValues) is called by SwiftUI's rendering engine at draw time with the
  correct dark/light context.

Neither feature requires Swift from the developer. Both are purely additive — no existing widget
behaviour changes.

Developer experience

import { Voltra } from 'voltra'

export const MyWidget = () => (
  <Voltra.VStack style={{ flex: 1, padding: 16 }}>
    <Voltra.Text
      widgetAccentable
      style={{ fontSize: 22, color: 'light-dark(#111111, #eeeeee)' }}
    >
      Adaptive text
    </Voltra.Text>
  </Voltra.VStack>
)

What was built

Artifact	Location	Purpose
widgetAccentable prop	packages/ios/src/jsx/baseProps.tsx, packages/core/src/payload/short-names.ts	New wa short name; applies .widgetAccentable() SwiftUI modifier
JSColorParser.parseLightDarkComponents()	packages/voltra/ios/ui/Style/JSColorParser.swift	Parses light-dark(…) into (light: Color, dark: Color) pair
LightDarkForeground: ShapeStyle	packages/voltra/ios/ui/Style/TextStyle.swift	Adaptive ShapeStyle — resolve(in:) picks correct Color at draw time
StyleConverter update	packages/voltra/ios/ui/Style/StyleConverter.swift	Routes light-dark() strings to lightDarkColors field
VoltraText update	packages/voltra/ios/ui/Views/VoltraText.swift	Uses foregroundStyle(LightDarkForeground(…)) when lightDarkColors is set
XCTest suite	packages/voltra/ios/Tests/JSColorParserTests.swift	118-line test file covering light-dark paths and existing smoke tests
Example widget	example/widgets/ios/IosWeatherWidget.tsx	Verification badges for manual testing

Exit criterion

• widgetAccentable prop serialised as wa short name and .widgetAccentable() applied in renderer — implementation complete; visual effect requires real device verification (StandBy dimmed/night view triggers vibrant/accented rendering; simulator cannot replicate this)
• light-dark() parsing implemented; adaptive LightDarkForeground: ShapeStyle wired into VoltraText — implementation complete; visual adaptation requires real device verification (iOS simulator does not re-render widgets on appearance toggle — confirmed with native Calendar widget as baseline)

Test plan

• Build in Xcode — BUILD SUCCEEDED
• Real device: StandBy dimmed/night view → widgetAccentable elements receive accent tint
• Real device: toggle dark/light mode in Settings → light-dark() text color adapts