refactor: split SherpaOnnxManager and extract magic numbers (P0-2, P1-4)

Split SherpaOnnxManager.swift (803 lines, 5 responsibilities) into focused components:
- ModelPathResolver: resolves local model paths and checks existence
- ModelDownloader: handles downloading from ModelScope/GitHub with fallback
- ModelExtractionService: handles tar.bz2 extraction
- SherpaOnnxManager: thin orchestrator delegating to the above

Extract magic numbers as named constants in:
- ChineseSpellingCorrector: logit/softmax thresholds, correction ratio limit
- DualEngineASR: flush silence sample count
- QwenASRRecognizer: chunk_sec, rollback, max_new_tokens, segment_sec
- ASREngine: flush silence sample count
- SherpaOnnxOnlineRecognizer: sample rate, feature dim, endpoint detection rules

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ZhaoChaoqun

Sources/ASREngine.swift

@@ -88,6 +88,15 @@ class StreamingParaformerEngine: ASREngine {
 
 /// QwenASR 流式引擎：模拟流式（chunk + rollback）
 class QwenASREngine: ASREngine {
+
+    // MARK: - Constants
+
+    /// 结束时推送的最小静音样本数（0.1s × 16kHz = 1600 samples）
+    /// 用于刷新尾部不足一个 chunk 的音频，不会导致 decoder hallucinate
+    private static let flushSilenceSampleCount = 1600
+
+    // MARK: - Properties
+
     private let recognizer: ASRStreamRecognizing
     private let recognitionQueue: DispatchQueue
 
@@ -127,7 +136,7 @@ class QwenASREngine: ASREngine {
             // 推送极少量 silence（0.1s）+ finalize，让 Rust 处理尾部不足一个 chunk 的音频
             // 并 commit rollback 窗口内的 token。0.1s silence 不会导致 decoder hallucinate，
             // 而之前的 1s silence 在长音频上导致 decoder 重复之前的内容。
-            let minimalSilence = [Float](repeating: 0.0, count: 1600)
+            let minimalSilence = [Float](repeating: 0.0, count: Self.flushSilenceSampleCount)
             _ = self.recognizer.pushAudio(samples: minimalSilence, finalize: true)
 
             let result = self.recognizer.getResult()

Sources/ChineseSpellingCorrector.swift

@@ -8,6 +8,26 @@ private let logger = Logger(subsystem: "com.typeless.app", category: "CSC")
 /// 使用 ONNX Runtime C API 加载 macbert4csc-base-chinese INT8 模型，
 /// 对 ASR 输出的中文文本进行同音字/近音字纠错。
 class ChineseSpellingCorrector {
+
+    // MARK: - Constants
+
+    /// ONNX Runtime intra-op 线程数
+    private static let ortIntraOpThreads: Int32 = 2
+
+    /// BERT 最大序列长度
+    private static let maxSequenceLength = 512
+
+    /// logit 差值阈值：预测 token 与原始 token 的 logit 差必须大于此值才执行替换
+    private static let logitDiffThreshold: Float = 5.0
+
+    /// softmax top-1 概率阈值：预测 token 的 softmax 概率必须大于此值才执行替换
+    private static let softmaxProbThreshold: Float = 0.9
+
+    /// 纠正比例上限：纠正的中文字符占比超过此值时丢弃所有纠正（防止上下文异常导致误纠）
+    private static let maxCorrectionRatio: Float = 0.2
+
+    // MARK: - Properties
+
     private let api: UnsafePointer<OrtApi>
     private var env: OpaquePointer?           // OrtEnv*
     private var session: OpaquePointer?       // OrtSession*
@@ -79,7 +99,7 @@ class ChineseSpellingCorrector {
         }
 
         // 设置线程数
-        _ = api.pointee.SetIntraOpNumThreads(sessionOptionsPtr, 2)
+        _ = api.pointee.SetIntraOpNumThreads(sessionOptionsPtr, Self.ortIntraOpThreads)
 
         // 创建 Session
         var sessionPtr: OpaquePointer? = nil
@@ -131,7 +151,7 @@ class ChineseSpellingCorrector {
         let seqLen = inputIds.count
 
         guard seqLen > 2 else { return text }  // 只有 [CLS] 和 [SEP]
-        guard seqLen <= 512 else {
+        guard seqLen <= Self.maxSequenceLength else {
             logger.warning("文本过长 (\(seqLen) tokens)，跳过 CSC")
             return text
         }
@@ -242,17 +262,17 @@ class ChineseSpellingCorrector {
                 let originalLogit = logitsPtr[logitsOffset + Int(originalId)]
                 let logitDiff = maxVal - originalLogit
 
-                // 条件 1：logit 差值必须足够大（提高到 5.0，防止低置信度替换）
-                guard logitDiff > 5.0 else { continue }
+                // 条件 1：logit 差值必须足够大，防止低置信度替换
+                guard logitDiff > Self.logitDiffThreshold else { continue }
 
-                // 条件 2：计算 softmax 概率，top-1 概率必须 > 0.9
+                // 条件 2：计算 softmax 概率，top-1 概率必须足够高
                 // 为数值稳定性，对 logits 减去 maxVal 后再 softmax
                 var expSum: Float = 0
                 for j in 0..<vocabSize {
                     expSum += exp(logitsPtr[logitsOffset + j] - maxVal)
                 }
                 let topProb = 1.0 / expSum  // exp(0) / expSum since maxVal - maxVal = 0
-                guard topProb > 0.9 else { continue }
+                guard topProb > Self.softmaxProbThreshold else { continue }
 
                 let correctedChar = tokenizer.decode([maxIdx])
                 if !correctedChar.isEmpty && correctedChar.count == 1 {
@@ -267,7 +287,7 @@ class ChineseSpellingCorrector {
         // Sanity check：如果纠正比例超过 20%，说明输入可能有上下文问题，丢弃纠正
         if correctionCount > 0 && chineseCharCount > 0 {
             let correctionRatio = Float(correctionCount) / Float(chineseCharCount)
-            if correctionRatio > 0.2 {
+            if correctionRatio > Self.maxCorrectionRatio {
                 logger.warning("CSC 纠正比例过高 (\(correctionCount)/\(chineseCharCount) = \(String(format: "%.0f%%", correctionRatio * 100)))，丢弃纠正结果")
                 return text
             }

Sources/DualEngineASR.swift

@@ -17,6 +17,12 @@ private let logger = Logger(subsystem: "com.typeless.app", category: "DualEngine
 /// - `paraformerEngine` 使用自己内部的 `recognitionQueue`，完全独立
 class DualEngineASR: ASREngine {
 
+    // MARK: - Constants
+
+    /// 结束时推送的最小静音样本数（0.1s × 16kHz = 1600 samples）
+    /// 用于刷新 QwenASR 尾部不足一个 chunk 的音频
+    private static let flushSilenceSampleCount = 1600
+
     // MARK: - 子引擎
 
     private let paraformerEngine: StreamingParaformerEngine
@@ -63,7 +69,7 @@ class DualEngineASR: ASREngine {
 
             // 推送极少量 silence（0.1s）+ finalize，让 Rust 处理尾部不足一个 chunk 的音频
             // 并 commit rollback 窗口内的 token
-            let minimalSilence = [Float](repeating: 0.0, count: 1600)
+            let minimalSilence = [Float](repeating: 0.0, count: Self.flushSilenceSampleCount)
             _ = self.qwenRecognizer.pushAudio(samples: minimalSilence, finalize: true)
 
             let result = self.qwenRecognizer.getResult()