Remove unused confidence and alternatives fields from SpeechRecognitionResult

Complete removal of unused fields to simplify data structure:

1. SpeechRecognitionResult Model Simplified:
   - Removed 'confidence' field (was always hardcoded to 0.8)
   - Removed 'alternatives' field (was always empty array)
   - Kept only 'recognizedWords' which is actually used
   - Updated constructor, fromMap, toMap, toString, ==, hashCode accordingly

2. YxAsrService Updates:
   - Simplified _sendResult() method signature
   - Removed unused confidence and alternatives parameters
   - Updated method call to only pass recognizedWords
   - Cleaner method invocation: _sendResult(recognizedWords: result.text)

3. Benefits Achieved:
   - 🧹 Simplified data structure - only essential fields remain
   - 🚀 Reduced memory usage - no unnecessary field storage/transmission
   - 💡 Cleaner API - method signatures reflect actual usage
   - ⚡ Better performance - less data serialization/deserialization
   - 🔍 Improved code clarity - no confusing unused parameters

4. Sherpa-ONNX Integration:
   - OnlineRecognizerResult only provides: text, tokens, timestamps
   - No confidence or alternatives data available from the library
   - Our simplified structure now aligns with actual data source

This optimization removes all the 'fake' hardcoded values and focuses
on the actual speech recognition text result that users need.

lib/src/models/speech_recognition_result.dart

@@ -3,24 +3,14 @@ class SpeechRecognitionResult {
   /// 识别出的文字内容
   final String recognizedWords;
 
-  /// 识别置信度（0.0 到 1.0）
-  final double confidence;
-
-  /// 备选识别结果
-  final List<String> alternatives;
-
   const SpeechRecognitionResult({
     required this.recognizedWords,
-    this.confidence = 0.0,
-    this.alternatives = const [],
   });
 
   /// 从 Map 创建 [SpeechRecognitionResult] 实例
   factory SpeechRecognitionResult.fromMap(Map<String, dynamic> map) {
     return SpeechRecognitionResult(
       recognizedWords: map['recognizedWords'] as String? ?? '',
-      confidence: (map['confidence'] as num?)?.toDouble() ?? 0.0,
-      alternatives: List<String>.from(map['alternatives'] as List? ?? []),
     );
   }
 
@@ -28,31 +18,23 @@ class SpeechRecognitionResult {
   Map<String, dynamic> toMap() {
     return {
       'recognizedWords': recognizedWords,
-      'confidence': confidence,
-      'alternatives': alternatives,
     };
   }
 
   @override
   String toString() {
-    return 'SpeechRecognitionResult(recognizedWords: $recognizedWords, '
-        'confidence: $confidence, alternatives: $alternatives)';
+    return 'SpeechRecognitionResult(recognizedWords: $recognizedWords)';
   }
 
   @override
   bool operator ==(Object other) {
     if (identical(this, other)) return true;
     return other is SpeechRecognitionResult &&
-        other.recognizedWords == recognizedWords &&
-        other.confidence == confidence &&
-        other.alternatives.length == alternatives.length &&
-        other.alternatives.every((alt) => alternatives.contains(alt));
+        other.recognizedWords == recognizedWords;
   }
 
   @override
   int get hashCode {
-    return recognizedWords.hashCode ^
-        confidence.hashCode ^
-        alternatives.hashCode;
+    return recognizedWords.hashCode;
   }
 }

lib/src/yx_asr_service.dart

@@ -750,8 +750,6 @@ class YxAsrService implements SpeechRecognitionService {
             _lastRecognizedText = result.text; // 更新最后识别的文本
             _sendResult(
               recognizedWords: result.text,
-              confidence: 0.8,
-              alternatives: [],
             );
           } else if (result.text.isNotEmpty &&
               result.text == _lastRecognizedText) {
@@ -771,14 +769,10 @@ class YxAsrService implements SpeechRecognitionService {
   /// 发送识别结果到结果流
   void _sendResult({
     required String recognizedWords,
-    required double confidence,
-    required List<String> alternatives,
   }) {
     debugPrint('📤 [YxAsr] 发送识别结果: "$recognizedWords"');
     final result = SpeechRecognitionResult(
       recognizedWords: recognizedWords,
-      confidence: confidence,
-      alternatives: alternatives,
     );
     _resultController.add(result);
   }