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/UtteranceTranscriptGuard.swift

@@ -0,0 +1,35 @@
+// UtteranceTranscriptGuard.swift
+// OSGKeyboard · Shared
+//
+// Chooses the best available transcript when pipelined final text may have
+// dropped a weak tail segment.
+
+import Foundation
+
+public enum UtteranceTranscriptGuard {
+    /// Partial must exceed final by at least this many characters to win.
+    public static let defaultPartialAdvantage = 8
+
+    /// Prefer `stitchedFinal` unless empty or clearly shorter than the live
+    /// partial snapshot taken at mic stop.
+    public static func resolve(
+        stitchedFinal: String,
+        partialSnapshot: String,
+        minimumPartialAdvantage: Int = defaultPartialAdvantage
+    ) -> String {
+        let final = stitchedFinal.trimmingCharacters(in: .whitespacesAndNewlines)
+        let partial = partialSnapshot.trimmingCharacters(in: .whitespacesAndNewlines)
+
+        if final.isEmpty { return partial }
+        if partial.isEmpty { return final }
+
+        if partial.count >= final.count + minimumPartialAdvantage {
+            FlowPipelineDiagnostics.logTranscriptGuardUsedPartial(
+                finalLength: final.count,
+                partialLength: partial.count
+            )
+            return partial
+        }
+        return final
+    }
+}