feat: setting fallback fonts

[mfazekas]

Adds RiveFonts — font fallback API for react-native. Two-phase design: load fonts into opaque handles, then configure a weight-based fallback map. System font fallback is opt-in via a 'default' sentinel.

const regular = await RiveFonts.loadFont({ uri: 'https://…/font.ttf' });
const bold = await RiveFonts.loadFont('kanit_regular.ttf');
const cjk = await RiveFonts.loadFont({ name: 'Noto Sans CJK' });

// Custom fonts only — no system fallback
await RiveFonts.setFallbackFonts({
  default: [regular, cjk],
  700: [bold, cjk],
});

// Custom fonts + system fallback (explicit opt-in)
await RiveFonts.setFallbackFonts({
  default: [regular, cjk, 'default'],
});

// Reset
await RiveFonts.clearFallbackFonts();

How to test

Open the Font Fallback exerciser in the example app (Exercisers > Font Fallback).

1. Custom fonts only — select fonts, mount without "System Fallback" toggle — unsupported scripts show tofu.

2. Custom fonts + system fallback — select fonts, toggle "System Fallback" on, mount — unsupported scripts render via platform default.

3. Font ordering — toggles show selection order (1, 2, 3…) since fallback priority follows insertion order.

iOS	Android

iOS	Android

[edgarrii]

@mfazekas thank you!

[dskuza]

From the Apple side, this looks good to me. The one thing I would question (but it could be a followup) is whether we can add constants for the weights. You can see what Apple uses by viewing UIFont.Weight documentation, but it's also similar to this name mapping.

[dskuza]

Is this the way of "guaranteeing" there's at least one fallback font in use (if all else fail)? We don't do this on iOS (there's only a default if you don't provide an explicit list). Not saying this is wrong, just something to call out.

[HayesGordon]

LGTM!