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>

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 - **Flow ASR pipelining**: shorter first chunk (2.5s) and 5s follow-ups so short utterances start on-device recognition while still recording; session-level ASR warmup and format cache reuse; live partials mirrored to the keyboard transcript line. / **Flow ASR 流水线**：首块 2.5 秒、后续 5 秒，短句录音期间即开始端侧识别；会话级 ASR 预热与格式缓存复用；实时 partial 同步到键盘转写行。
+- **Flow tail drain**: after mic stop, capture drains trailing PCM (silence-detected, capped) before finishing the ASR stream; host finalize awaits drain; short final chunks re-transcribed with prior overlap; stitcher safe fallback when overlap merge would drop content. / **Flow 尾音排空**：停止录音后先排空尾部 PCM（静音检测 + 上限）再结束 ASR 流；主 App finalize 等待排空；过短末块与上一块 overlap 合并重识别；拼接误删时回退为安全合并。
 
 ## [0.4.0] - 2026-07-05