fix(flow): improve tail capture and final chunk ASR recovery

- Add FlowUtteranceEndCoordinator with 350ms silence drain and 150ms post-roll
- Extend FinalChunkRecovery for short/empty final chunks in chunked pipeline
- Snapshot partial at mic stop and guard final transcript in FlowSessionManager
- Unify tail drain presets (iosFlow/macMLX) and expand diagnostics

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

OSGKeyboardShared/Utilities/FlowPipelineDiagnostics.swift

@@ -9,10 +9,22 @@ 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)"
+            "tailDrain duration=\(report.drainDurationSeconds, format: .fixed(precision: 2))s " +
+            "postRoll=\(report.postRollDurationSeconds, format: .fixed(precision: 2))s " +
+            "silenceEnd=\(report.endedBySilence) tailSamples=\(report.tailSampleCount)"
         )
     }
 
+    public static func logTranscriptGuardUsedPartial(finalLength: Int, partialLength: Int) {
+        OSGLog.flow.warning(
+            "transcriptGuard partial preferred finalLen=\(finalLength) partialLen=\(partialLength)"
+        )
+    }
+
+    public static func logFinalChunkRecovery(action: String, chunkIndex: Int) {
+        OSGLog.asr.info("finalChunkRecovery \(action) chunk=\(chunkIndex)")
+    }
+
     public static func logChunkFinalize(
         chunkCount: Int,
         lastChunkSamples: Int,