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>

OSGKeyboardTests/ChunkedUtterancePipelineTests.swift

@@ -87,6 +87,34 @@ final class ChunkedUtterancePipelineTests: XCTestCase {
         XCTAssertFalse(success.text.isEmpty)
         XCTAssertEqual(success.chunkWarnings.count, 1)
     }
+
+    func testPipelineRetranscribesShortFinalChunkWithPriorOverlap() async {
+        let config = FlowUtteranceChunkConfig(
+            maxChunkDurationSeconds: 0.05,
+            overlapDurationSeconds: 10,
+            pauseExtensionMaxSeconds: 0,
+            pauseRMSThreshold: 0.02,
+            minFinalChunkDurationSeconds: 0.05,
+            sampleRate: 1_000
+        )
+        let asr = ShortFinalMergeStubASR()
+        let pipeline = ChunkedUtterancePipeline(
+            asr: asr,
+            locale: Locale(identifier: "zh-Hans"),
+            config: config
+        )
+
+        let (stream, continuation) = AsyncStream<AudioBufferSnapshot>.makeStream()
+        continuation.yield(AudioBufferSnapshot(samples: [Float](repeating: 0.1, count: 80), sampleRate: 1_000))
+        continuation.yield(AudioBufferSnapshot(samples: [Float](repeating: 0.1, count: 20), sampleRate: 1_000))
+        continuation.finish()
+
+        let outcome = await pipeline.transcribe(stream: stream) { _ in }
+        guard case .success(let success) = outcome else {
+            return XCTFail("expected success, got \(outcome)")
+        }
+        XCTAssertTrue(success.text.contains("merged"))
+    }
 }
 
 private struct FailingSecondChunkASR: ASRService, @unchecked Sendable {
@@ -114,3 +142,32 @@ private struct FailingSecondChunkASR: ASRService, @unchecked Sendable {
         return .success("seg\(samples.count)")
     }
 }
+
+private struct ShortFinalMergeStubASR: ASRService, @unchecked Sendable {
+    private let callIndex = OSAllocatedUnfairLock(initialState: 0)
+
+    func transcribe(
+        stream: AsyncStream<AudioBufferSnapshot>,
+        locale: Locale
+    ) -> AsyncStream<ASREvent> {
+        AsyncStream { $0.finish() }
+    }
+
+    func cancel() {}
+
+    func transcribeChunk(samples: [Float], locale: Locale) async -> ASRChunkResult {
+        _ = locale
+        let current = callIndex.withLock { state in
+            let value = state
+            state += 1
+            return value
+        }
+        if current == 0 {
+            return .success("head")
+        }
+        if samples.count > 20 {
+            return .success("merged-tail")
+        }
+        return .success("short")
+    }
+}