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>

OSGKeyboardTests/UtteranceTranscriptGuardTests.swift

@@ -0,0 +1,32 @@
+// UtteranceTranscriptGuardTests.swift
+// OSGKeyboardTests
+
+import XCTest
+@testable import OSGKeyboardShared
+
+final class UtteranceTranscriptGuardTests: XCTestCase {
+
+    func testResolvePrefersPartialWhenClearlyLonger() {
+        let resolved = UtteranceTranscriptGuard.resolve(
+            stitchedFinal: "今天天气很好",
+            partialSnapshot: "今天天气很好，我们一起去公园吧"
+        )
+        XCTAssertEqual(resolved, "今天天气很好，我们一起去公园吧")
+    }
+
+    func testResolveKeepsFinalWhenPartialIsNotLonger() {
+        let resolved = UtteranceTranscriptGuard.resolve(
+            stitchedFinal: "今天天气很好，我们一起去公园吧",
+            partialSnapshot: "今天天气很好"
+        )
+        XCTAssertEqual(resolved, "今天天气很好，我们一起去公园吧")
+    }
+
+    func testResolveUsesPartialWhenFinalEmpty() {
+        let resolved = UtteranceTranscriptGuard.resolve(
+            stitchedFinal: "",
+            partialSnapshot: "最后一段 partial"
+        )
+        XCTAssertEqual(resolved, "最后一段 partial")
+    }
+}