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>

OSGKeyboardShared/Utilities/FlowPipelineDiagnostics.swift

@@ -0,0 +1,34 @@
+// FlowPipelineDiagnostics.swift
+// OSGKeyboard · Shared
+//
+// Structured Flow pipeline metrics for Console.app filtering.
+
+import Foundation
+import os
+
+public enum FlowPipelineDiagnostics {
+    public static func logDrain(_ report: FlowCaptureDrainReport) {
+        OSGLog.flow.info(
+            "tailDrain duration=\(report.drainDurationSeconds, format: .fixed(precision: 2))s " +
+            "silenceEnd=\(report.endedBySilence) tailSamples=\(report.tailSampleCount)"
+        )
+    }
+
+    public static func logChunkFinalize(
+        chunkCount: Int,
+        lastChunkSamples: Int,
+        stitchedLength: Int,
+        chunkWarnings: Int
+    ) {
+        OSGLog.flow.info(
+            "chunkPipeline chunks=\(chunkCount) lastChunkSamples=\(lastChunkSamples) " +
+            "stitchedLen=\(stitchedLength) warnings=\(chunkWarnings)"
+        )
+    }
+
+    public static func logStitcherSafeFallback(naiveLength: Int, mergedLength: Int) {
+        OSGLog.asr.warning(
+            "stitcher safe fallback naive=\(naiveLength) merged=\(mergedLength)"
+        )
+    }
+}