iOS: emitOnBrownfieldMessage: can crash with std::bad_function_call when native posts before JS requires the TurboModule (cold-start race)

`ReactNativeBrownfieldModule.mm`'s `+emitMessageFromNative:` only checks that the Obj-C singleton exists before calling `emitOnBrownfieldMessage:`:

```objc
+ (void)emitMessageFromNative:(NSString *)message {
if (_sharedInstance) {
[_sharedInstance emitOnBrownfieldMessage:@{ @"text": message }];
} else {
NSLog(@"ReactNativeBrownfieldModule is not initialized, dropping message");
}
}
```

`_sharedInstance` is set in `-init`, which runs as soon as the bridge/TurboModuleManager instantiates the Obj-C module. But the codegen `SpecBase`'s `_eventEmitterCallback` (the `std::function` that `emitOnBrownfieldMessage:` invokes) is only populated inside the C++ `SpecJSI` constructor, which only runs when `getTurboModule:` fires — i.e. when JS actually evaluates `TurboModuleRegistry.getEnforcing('ReactNativeBrownfield')`.

If a host app calls `ReactNativeBrownfield.shared.postMessage(...)` (→ posts `BrownfieldMessageToJSNotification` → `handleNativeToJSMessage:` → `emitMessageFromNative:`) after the Obj-C module exists but before JS has required the module — e.g. right as a hosting view controller's `viewDidAppear` fires on a cold app launch, racing bundle evaluation — `_eventEmitterCallback` is still empty, and calling it throws `std::bad_function_call`, crashing the whole app (uncaught C++ exception).

Confirmed still present on `main` (4.0.0) as of 2026-07-06 — `emitMessageFromNative:`'s guard is unchanged from what's below (also reproduced against 3.3.0 in our own app):

```objc
- (void)handleNativeToJSMessage:(NSNotification *)notification {
NSString *message = notification.userInfo[@"message"];
if (message) {
[ReactNativeBrownfieldModule emitMessageFromNative:message];
}
}
```

### Suggested fixes
- Wrap the `_eventEmitterCallback` invocation in a readiness check (or try/catch) inside the generated `emitOnBrownfieldMessage:`, or
- Expose a callback/promise so host apps can gate native→JS `postMessage` calls on "TurboModule ready" rather than just "bundle loaded" (the closest signal `startReactNative(onBundleLoaded:)` currently offers).

### Repro
Timing-dependent — we haven't produced a deterministic repro case, but the code path is unambiguous from source, and we hit it in production (Crashlytics, `std::__1::bad_function_call: std::exception`, 100% of occurrences within the first second of a session). Happy to share a sanitized stack trace if useful.

### Environment
- `@callstack/react-native-brownfield`: 3.3.0 (also read against `main`/4.0.0 source)
- iOS 26.x, React Native new architecture (TurboModules/Fabric)

Inizia in ReactNativeBrownfieldModule.mm con -handleNativeToJSMessage: e +emitMessageFromNative:, quindi segui emitOnBrownfieldMessage: fino all’inizializzazione del callback di SpecBase generato. Riproduci o strumenta un post nativo prima che JS richieda il TurboModule e verifica che questo percorso di avvio a freddo non vada più in crash quando il callback non è disponibile.

Scritto dal modello di indicizzazione a partire dal testo della issue.