fix(flow): drain trailing audio and harden chunked ASR pipeline

- P0/P1: tail-drain state machine in FlowContinuousCapture with converter
  flush; FlowSessionManager awaits drain before finalize
- P2: FlowCaptureTailDrain policy/tracker, pipeline diagnostics, unit tests
- P3: short final chunk merged re-transcription in ChunkedUtterancePipeline
- P4: UtteranceTranscriptStitcher composedSafely fallback; preview path parity
  in LiveDictationController

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

OSGKeyboard/Services/FlowDiagnostics.swift

@@ -11,4 +11,8 @@ enum FlowDiagnostics {
     static func log(_ message: String) {
         OSGLog.flow.info("\(message, privacy: .public)")
     }
+
+    static func logDrain(_ report: FlowCaptureDrainReport) {
+        FlowPipelineDiagnostics.logDrain(report)
+    }
 }

OSGKeyboard/Services/FlowSessionManager.swift

@@ -485,20 +485,20 @@ final class FlowSessionManager: ObservableObject {
 
         // Close the mic gate first, then mark processing before dropping the
         // recording flag so the poll loop cannot start a second utterance.
-        capture.endUtterance()
         FlowSessionBridge.setRecordingState(.processing)
         isUtteranceRecording = false
         isUtteranceProcessing = true
 
-        // Do NOT cancel `asrTask` or `asr` — the preview pipeline relies on
-        // the consumer staying alive until `.final` lands (see
-        // `PreviewASRControllerStateTests`).
+        // Do NOT cancel `asrTask` or `asr` — drain trailing PCM, then finalize.
 
         finalizeTask?.cancel()
         finalizeTask = Task { @MainActor [weak self] in
-            await self?.finalizeUtterance()
+            guard let self else { return }
+            let drainReport = await self.capture.endUtteranceAndDrain()
+            FlowDiagnostics.logDrain(drainReport)
+            await self.finalizeUtterance()
         }
-        debug("utterance stopped, finalizing")
+        debug("utterance stopped, draining tail")
     }
 
     private func abortUtterance() {