fix: add TextDecoder/TextEncoder polyfill for React Native

[devin-ai-integration]

Description

Fixes ReferenceError: Property 'TextDecoder' doesn't exist reported by a client using CrossmintEmbeddedCheckout in a bare React Native 0.76.5 app on Android (Pylon ticket).

React Native's JS engines (Hermes/JSC) don't include TextDecoder/TextEncoder by default. The SDK uses these in API key validation (validateAPIKey.ts:53,76) and HPKE encryption (hpke.ts:78,82), causing an immediate crash on Android.

This adds fast-text-encoding as a polyfill to @crossmint/client-sdk-rn-window, following the same pattern established for crypto.getRandomValues via react-native-get-random-values.

⚠️ Items for reviewer attention

1. Polyfill load timing — The polyfill is loaded via polyfills.ts → RNWebViewTransport.ts. If TextDecoder is needed before the transport is initialized (e.g. early API key validation), it may still fail. This matches the existing react-native-get-random-values pattern, but worth considering whether polyfills should be re-exported from the package entry point instead.

Link to Devin run | Requested by @xmint-guille

Test plan

• Lint passes (pnpm lint)
• All unit tests pass (pnpm test:vitest, 11/11 suites)
• pnpm build:libs succeeds
• Not tested on a real React Native Android device — the fix can only be fully validated on bare RN 0.76+ on Android, which is the affected environment. Unit tests run in Node.js where TextDecoder is already available natively.

Package updates

• Added fast-text-encoding@^1.0.6 to @crossmint/client-sdk-rn-window dependencies
• Changeset added for patch bump on @crossmint/client-sdk-rn-window

[changeset-bot]

🦋 Changeset detected

Latest commit: 7bf1479

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@crossmint/client-sdk-rn-window	Patch
expo-demo	Patch
@crossmint/client-sdk-react-native-ui	Patch

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[devin-ai-integration]

Original prompt from Guille

check this ticket

i made claude code to analyze it and propose a diagnosis. evaluate his analysis, and in case useful propose a fix in a draft pr. don't tag nobody in the pr review.

# TextDecoder Issue in Crossmint React Native SDK - Diagnosis

## Issue Summary

Client reported: **ReferenceError: Property 'TextDecoder' doesn't exist** when using `CrossmintEmbeddedCheckout` in a bare React Native 0.76.5 app on Android.

**Client's Workaround**: Installing `text-encoding` library and importing it globally.

---

## Root Cause Analysis

### 1. **Environment Limitation**
- `TextDecoder` and `TextEncoder` are Web APIs that are **not available** in React Native's JavaScript environment by default, especially on Android
- These APIs are standard in browsers and Node.js, but React Native uses JavaScriptCore (iOS) or Hermes (Android) which don't include them

### 2. **Where TextDecoder is Used in the SDK**

The Crossmint SDK uses `TextDecoder` in two critical places:

#### a) **API Key Validation** (`@crossmint/common-sdk-base`)
File: `packages/common/base/src/apiKey/validateAPIKey.ts:53`

```typescript
function decodeAPIKeyAndGetParts(apiKey: string, prefix: APIKeyPrefix) {
    const base58EncodedPart = apiKey.slice(`${prefix}_`.length);
    const decodedPartUint8 = base58.decode(base58EncodedPart);
    const decodedPartString = new TextDecoder().decode(decodedPartUint8);  // ← FAILS HERE
    // ...
}

Also uses TextEncoder at line 76:

const keyDataBytes = new TextEncoder().encode(keyData);

b) HPKE Encryption (@crossmint/client-signers-cryptography)

File: packages/client/signers-cryptography/src/algorithms/asymmetric/hpke.ts:82

private deserialize<T extends EncryptablePayload>(data: ArrayBuffer): T {
    return JSON.parse(new TextDecoder().decode(data)) as T;  // ← FAILS HERE
}

Also uses `... (5396 chars truncated...)

</details>

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