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>

OSGKeyboardMac/MacMLXLiveCapture.swift

@@ -7,11 +7,7 @@ import Foundation
 import os
 
 enum MacMLXLiveCapture {
-    private static let tailDrainPolicy = FlowCaptureTailDrainPolicy(
-        silenceRMSThreshold: 0.015,
-        silenceDurationSeconds: 0.35,
-        maxDrainSeconds: 0.75
-    )
+    private static let tailDrainPolicy = FlowCaptureTailDrainPolicy.macMLX
 
     /// Runs MLX streaming ASR until `finishSignal` fires, then tail-drains and finalizes.
     static func run(