feat(theme): Add medium and high contrast theme

[hristototov]

Motivation

The theme only generates the MD3 standard contrast level. MD3 defines three levels, and the higher two exist for accessibility: they raise the contrast between text and background so the UI stays readable for users with low vision and in bright light. Without them, apps built with Paper have no supported way to meet the raised contrast ratios.

This adds the medium and high schemes for light and dark, and a contrast prop on PaperProvider to select them. standard stays the default and its colours do not change. It uses scripts/generate-contrast-tokens.ts

Measured against the baseline #6750A4 seed:

scheme	primary tone	onPrimaryContainer tone
light / standard	40.1	10.1
light / medium	26.7	100.0
light / high	13.3	86.9
dark / high	96.7	23.8

API

<PaperProvider contrast="high">
  <App />
</PaperProvider>

Related issue

Closes #5092

Test plan

Automated:

• src/theme/__tests__/contrast.test.ts checks 17 text/background role pairs across both modes and both new levels, 64 pairs in total, and asserts >= 4.5 for medium and >= 7.0 for high. It also asserts that LightTheme and DarkTheme colours are unchanged.
• src/core/__tests__/PaperProvider.test.tsx covers the contrast prop, the standard default, prop precedence over theme.contrast, and that a provider with only contrast set still follows the system color scheme.
• No component snapshot colours changed. The only snapshot diff is the new contrast field on the serialized theme.

Manual, on an Android 12 emulator (API 31), using the new Contrast control in the example app drawer under Preferences:

1. Open the drawer, switch between Standard, Medium and High. Colours change at each step.
2. Toggle Dark Theme and repeat. All six combinations render correctly with no missing colours.
3. With "Use Dynamic Theme" on, note that Standard shows the wallpaper-based colours while Medium and High fall back to the contrast-correct scheme. This is the intended behaviour described above.
4. Restart the app. The selected level is restored from storage.

android-contrast.mov

ios-contrast.mov

[JKobrynski]

These don't match MD3's published contrast schemes. On the color roles page, light/high primary is md.ref.palette.primary20 (#381E72), but this ships #312259; secondary, outline, outline-variant and on-surface-variant are off too, and surface-container isn't meant to change with contrast at all. Every tone MD3 asks for is already a key of ref/palette.ts - could roleToTone just gain medium and high maps, the way the old // extend with 'medium' | 'high' when those ship comment planned?

[JKobrynski]

Passing a theme that has its own colors silently overrides the contrast scheme, but theme.contrast still reports the level you asked for - so useTheme().contrast says "high" while the colours on screen are standard. Worth reconciling, given the docs tell people to read the level back off the theme.

[JKobrynski]

This builds a new theme object on every render, so the provider rebuilds the theme and every themed component re-renders with it. getTheme(isDarkMode, contrast) returns the cached one worth using that here?

[JKobrynski]

And we've got conflicts