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[FSSDK-13287] Make React Native polyfills optional and isolate optional requires (#1179)

## Summary
- **Polyfill packages are optional.** `fast-text-encoding` and `react-native-get-random-values` are loaded if installed. Both keep an existing global, so a native `TextEncoder` (Hermes in RN 0.74+) or an app's own polyfill (for example `react-native-quick-crypto`, imported in the app's entry file before the SDK) is used instead. If `TextEncoder` or `crypto.getRandomValues` is still missing, `index.react_native.ts` throws at import with an error that names the package to install.
- **Optional import shims in `lib/import.react_native/`.** There is one shim per optional package, named after the package: `fast-text-encoding.ts`, `react-native-get-random-values.ts` and `@react-native-async-storage/async-storage.ts`. Each one:
  - does its `require` inside `try/catch` while the module loads;
  - exports `isAvailable` plus the package's exports that the SDK uses, under the same names, which are `undefined` if the package is missing;
  - never throws. The importer decides what a missing package means.
- **Shims are built as separate files.** Rollup builds each shim to the same path under `dist/import.react_native/`, and the main React Native bundles load them by relative path. A small `resolveId` plugin keeps the shim imports out of the main bundles. The main bundles no longer contain any optional `require`, and the node, browser and universal bundles are unchanged.
- **AsyncStorage:** the compat helper (`ensureMultiGet`, which adds `multiGet` on v3) is merged into the async-storage shim. The shim exports the package's `default`, with `multiGet` added. The "Module not found" check moved to `AsyncStorageCache`. The CJS bundles read shim default exports through Rollup's `esModule` interop, applied only to shim imports.
- **Build check:** a Rollup plugin fails the build unless every built shim contains exactly one `require()`, of its own package. That catches imp... (continued)

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