feat(flow): add whole-utterance batch ASR fallback (P1)

- Accumulate utterance PCM in FlowContinuousCapture for batch retry
- Run full-utterance transcribeChunk when stitched final lags partial
- Refactor Mac MLX tail drain to shared FlowUtteranceEndCoordinator
- Add FlowUtterancePCMStore, UtteranceBatchFallbackPolicy, and tests

Co-authored-by: Rocky <hkgood@users.noreply.github.com>

OSGKeyboard/Services/FlowSessionManager.swift

@@ -66,6 +66,8 @@ final class FlowSessionManager: ObservableObject {
     private var lastFinal = ""
     /// Partial stitched text captured when the user stops recording.
     private var bestPartialSnapshot = ""
+    /// Full utterance PCM for batch ASR fallback after pipelined chunking.
+    private var utterancePCMSamples: [Float] = []
     private var chunkWarnings: [String] = []
     private var lastReadyTraceSignature = ""
     private var lastCommandFingerprint = ""
@@ -359,6 +361,7 @@ final class FlowSessionManager: ObservableObject {
         currentPartial = ""
         lastFinal = ""
         bestPartialSnapshot = ""
+        utterancePCMSamples = []
         chunkWarnings = []
         FlowSessionBridge.setHostReady(false)
     }
@@ -1105,6 +1108,7 @@ final class FlowSessionManager: ObservableObject {
         currentPartial = ""
         lastFinal = ""
         bestPartialSnapshot = ""
+        utterancePCMSamples = []
         chunkWarnings = []
 
         let localeId = store.localeId
@@ -1214,6 +1218,7 @@ final class FlowSessionManager: ObservableObject {
             guard let self else { return }
             let drainReport = await self.capture.endUtteranceAndDrain()
             FlowDiagnostics.logDrain(drainReport)
+            self.utterancePCMSamples = self.capture.consumeUtteranceSamples()
             await self.finalizeUtterance(
                 sessionId: drainingSessionId,
                 utteranceId: drainingUtteranceId,
@@ -1239,6 +1244,7 @@ final class FlowSessionManager: ObservableObject {
         currentPartial = ""
         lastFinal = ""
         bestPartialSnapshot = ""
+        utterancePCMSamples = []
         chunkWarnings = []
         currentUtteranceId = nil
         currentCommandSeq = 0
@@ -1266,6 +1272,7 @@ final class FlowSessionManager: ObservableObject {
         currentPartial = ""
         lastFinal = ""
         bestPartialSnapshot = ""
+        utterancePCMSamples = []
         chunkWarnings = []
         storeCurrentError(message, kind: kind)
         currentUtteranceId = nil
@@ -1289,6 +1296,7 @@ final class FlowSessionManager: ObservableObject {
         currentPartial = ""
         lastFinal = ""
         bestPartialSnapshot = ""
+        utterancePCMSamples = []
         chunkWarnings = []
         storeCurrentError(message, kind: kind)
         currentUtteranceId = nil
@@ -1348,6 +1356,14 @@ final class FlowSessionManager: ObservableObject {
         if text.isEmpty {
             text = currentPartial.trimmingCharacters(in: .whitespacesAndNewlines)
         }
+
+        if UtteranceBatchFallbackPolicy.shouldRunBatchFallback(
+            stitchedFinal: lastFinal,
+            partialSnapshot: bestPartialSnapshot
+        ), !utterancePCMSamples.isEmpty {
+            text = await runBatchASRFallback(currentText: text)
+        }
+        utterancePCMSamples = []
         guard !text.isEmpty else {
             let key = (asrTask?.isCancelled == true || Task.isCancelled)
                 ? "flow.error.recognitionInterrupted"
@@ -1441,6 +1457,7 @@ final class FlowSessionManager: ObservableObject {
         currentPartial = ""
         lastFinal = ""
         bestPartialSnapshot = ""
+        utterancePCMSamples = []
         chunkWarnings = []
         chunkedPipeline = nil
         debug("utterance finalized length=\(text.count)")
@@ -1569,6 +1586,49 @@ final class FlowSessionManager: ObservableObject {
         )
     }
 
+    /// Re-transcribe the full utterance PCM when pipelined chunking likely dropped tail text.
+    private func runBatchASRFallback(currentText: String) async -> String {
+        let samples = utterancePCMSamples
+        guard !samples.isEmpty else { return currentText }
+
+        let locale = SpeechLocaleResolver.resolve(store.localeId)
+        let stitched = lastFinal.trimmingCharacters(in: .whitespacesAndNewlines)
+        let partial = bestPartialSnapshot.trimmingCharacters(in: .whitespacesAndNewlines)
+
+        FlowDiagnostics.log(
+            "batch fallback start samples=\(samples.count) stitchedLen=\(stitched.count) partialLen=\(partial.count)"
+        )
+
+        let asrService = asr
+        let result = await Task.detached(priority: .userInitiated) { [asrService] in
+            await asrService.transcribeChunk(samples: samples, locale: locale)
+        }.value
+
+        switch result {
+        case .success(let batchText):
+            let trimmedBatch = batchText.trimmingCharacters(in: .whitespacesAndNewlines)
+            guard !trimmedBatch.isEmpty else { return currentText }
+            let resolved = UtteranceBatchFallbackPolicy.preferredTranscript(
+                batch: trimmedBatch,
+                stitchedFinal: stitched,
+                partialSnapshot: partial,
+                current: currentText
+            )
+            FlowPipelineDiagnostics.logBatchFallback(
+                sampleCount: samples.count,
+                stitchedLength: stitched.count,
+                partialLength: partial.count,
+                batchLength: trimmedBatch.count
+            )
+            return resolved
+        case .failure(let message):
+            FlowDiagnostics.log("batch fallback failed: \(message)")
+            return currentText
+        case .cancelled:
+            return currentText
+        }
+    }
+
     private func asrWaitTimeout() -> TimeInterval {
         // v0.2.0: local engine is iOS `SpeechAnalyzer` only, so the
         // previous Qwen3-specific timeout collapses into the shared