fix: address PR #474 review comments — 3 fixes in Flutter voice-agent bridge

- **QEF-1 (coderabbit, P0/Major, line 318)**: Pass `handle.address` across
  `Isolate.run` boundary instead of the raw `Pointer<Void>` and reconstruct it
  via `RacHandle.fromAddress(...)` inside `_processVoiceTurnInIsolate`. Matches
  the pattern already used by `dart_bridge_tts`, `dart_bridge_stt`,
  `dart_bridge_llm`, and `dart_bridge_vlm`; passing Dart-side pointers across
  isolate boundaries is unsafe.

- **QEF-3 (greptile, P2, line 109)**: Add a clarifying comment explaining why
  `_initFuture` is cleared *after* `completer.complete(_handle!)` — concurrent
  callers already holding a reference to `completer.future` still receive the
  value normally; new callers arriving after this line hit the `_handle != null`
  fast path.

- **QEF-4 (greptile outside-diff)**: Replace the plain `String` type on
  `VoiceAgentModelLoadedEvent.component` with a new typed enum
  `VoiceAgentComponent { stt, llm, tts }`, and update the three emit sites
  (`loadSTTModel`, `loadLLMModel`, `loadTTSVoice`) accordingly. Per the project's
  style guide ("never use strings directly"), the enum makes component typos a
  compile-time error rather than a silently-accepted string.

Not addressed:
- **QEF-2 (greptile, P2 — `sttResultFree` should be nullable)**: Already resolved
  on this branch via a different approach — `sttResultFree` was removed from
  `native_functions.dart` entirely. The STT transcription path runs inside
  `Isolate.run(...)` which cannot read main-isolate static state, so each spawned
  isolate resolves `rac_stt_result_free` inline. The comment block at
  `native_functions.dart:89-93` explicitly documents this decision.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sanchitmonga22

sdk/runanywhere-flutter/packages/runanywhere/lib/native/dart_bridge_voice_agent.dart

@@ -106,6 +106,10 @@ class DartBridgeVoiceAgent {
         _handle = handlePtr.value;
         _logger.info('Voice agent created with shared component handles');
         completer.complete(_handle!);
+        // Clear _initFuture after completing the completer so that concurrent
+        // callers that already hold a reference to completer.future receive the
+        // value normally. New callers arriving after this line hit the
+        // `_handle != null` fast path.
         _initFuture = null;
         return _handle!;
       } finally {
@@ -214,7 +218,8 @@ class DartBridgeVoiceAgent {
       }
 
       _logger.info('Voice agent STT model loaded: $modelId');
-      _eventController.add(const VoiceAgentModelLoadedEvent(component: 'stt'));
+      _eventController.add(const VoiceAgentModelLoadedEvent(
+          component: VoiceAgentComponent.stt));
     } finally {
       calloc.free(pathPtr);
       calloc.free(idPtr);
@@ -239,7 +244,8 @@ class DartBridgeVoiceAgent {
       }
 
       _logger.info('Voice agent LLM model loaded: $modelId');
-      _eventController.add(const VoiceAgentModelLoadedEvent(component: 'llm'));
+      _eventController.add(const VoiceAgentModelLoadedEvent(
+          component: VoiceAgentComponent.llm));
     } finally {
       calloc.free(pathPtr);
       calloc.free(idPtr);
@@ -264,7 +270,8 @@ class DartBridgeVoiceAgent {
       }
 
       _logger.info('Voice agent TTS voice loaded: $voiceId');
-      _eventController.add(const VoiceAgentModelLoadedEvent(component: 'tts'));
+      _eventController.add(const VoiceAgentModelLoadedEvent(
+          component: VoiceAgentComponent.tts));
     } finally {
       calloc.free(pathPtr);
       calloc.free(idPtr);
@@ -314,16 +321,22 @@ class DartBridgeVoiceAgent {
           'Voice agent not ready. Load models and initialize first.');
     }
 
-    // Run the heavy C++ processing in a background isolate
-    return Isolate.run(() => _processVoiceTurnInIsolate(handle, audioData));
+    // Capture handle address before entering isolate — passing the raw Pointer
+    // across an isolate boundary is unsafe; pass the address and reconstruct it.
+    final handleAddress = handle.address;
+    return Isolate.run(
+        () => _processVoiceTurnInIsolate(handleAddress, audioData));
   }
 
   /// Static helper for processing voice turn in an isolate.
   /// The C++ API expects raw audio bytes (PCM16), not float samples.
+  /// Must be static/top-level for Isolate.run().
   static Future<VoiceTurnResult> _processVoiceTurnInIsolate(
-    RacHandle handle,
+    int handleAddress,
     Uint8List audioData,
   ) async {
+    final handle = RacHandle.fromAddress(handleAddress);
+
     // Allocate native memory for audio data (raw PCM16 bytes)
     final audioPtr = calloc<Uint8>(audioData.length);
     final resultPtr = calloc<RacVoiceAgentResultStruct>();
@@ -555,9 +568,12 @@ class VoiceAgentInitializedEvent extends VoiceAgentEvent {
   const VoiceAgentInitializedEvent();
 }
 
+/// Component types that can emit a model-loaded event on the voice agent.
+enum VoiceAgentComponent { stt, llm, tts }
+
 /// Voice agent model loaded.
 class VoiceAgentModelLoadedEvent extends VoiceAgentEvent {
-  final String component; // 'stt', 'llm', or 'tts'
+  final VoiceAgentComponent component;
   const VoiceAgentModelLoadedEvent({required this.component});
 }