verify that example app runs fine with new arch enabled

[cameronvoell]

Add getArchitectureDiagnostics to verify new architecture support in the example app

• Adds a getArchitectureDiagnostics() method to Client, the JS module, and native modules on both iOS and Android that returns runtime info about the active React Native architecture (new arch vs. bridge, module access path, sync support).
• On Android, reads IS_NEW_ARCHITECTURE_ENABLED from the host app's BuildConfig via reflection, falling back to the module's own BuildConfig; on iOS, uses a compile-time RCT_NEW_ARCH_ENABLED flag set via XMTPReactNative.podspec.
• The example app calls Client.getArchitectureDiagnostics() on startup and logs a structured report; example/app.json sets newArchEnabled: true to exercise the new-arch path.
• XMTPArchitectureDiagnostics.ts defines shared types and a toArchitectureDiagnostics() helper that normalizes native diagnostics into a typed JS object.

^{Macroscope summarized d1599f8.}

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[cameronvoell]

• verify that example app runs fine with new arch enabled #775 👈 (View in Graphite)
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[cameronvoell]

(NOBRIDGE) LOG  Bridgeless mode is enabled
 (NOBRIDGE) LOG  Shims Injected:
 (NOBRIDGE) LOG    - nextTick
 INFO  
 💡 JavaScript logs will be removed from Metro in React Native 0.77! Please use React Native DevTools as your default tool. Tip: Type j in the terminal to open (requires Google Chrome or Microsoft Edge).
 (NOBRIDGE) LOG  === XMTP ARCHITECTURE DIAGNOSTICS ===
 (NOBRIDGE) LOG    Native new architecture flag (RCT_NEW_ARCH_ENABLED via xmtp-module): ENABLED
 (NOBRIDGE) LOG    XMTP module access path: JSI
 (NOBRIDGE) LOG    Supports synchronous native methods: YES
 (NOBRIDGE) LOG    Native module type: expo-module
 (NOBRIDGE) LOG    Native module class: XMTPModule
 (NOBRIDGE) LOG    Host app id: expo.modules.xmtpreactnativesdk.example
 (NOBRIDGE) LOG    Platform: ios
 (NOBRIDGE) LOG  =====================================
 (NOBRIDGE) LOG  XMTP is running in a new-architecture app and is being resolved through the JSI module registry.