feat(flow): add whole-utterance batch ASR fallback (P1)

- Accumulate utterance PCM in FlowContinuousCapture for batch retry
- Run full-utterance transcribeChunk when stitched final lags partial
- Refactor Mac MLX tail drain to shared FlowUtteranceEndCoordinator
- Add FlowUtterancePCMStore, UtteranceBatchFallbackPolicy, and tests

Co-authored-by: Rocky <hkgood@users.noreply.github.com>

CHANGELOG.md

@@ -9,6 +9,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 - **Flow tail ASR drop**: after mic stop, iOS Flow now uses a longer silence drain (350 ms), a fixed 150 ms post-roll, expanded final-chunk ASR recovery, and a partial transcript guard so weak trailing syllables are less likely to disappear from the result. / **Flow 尾音识别丢失**：松手后 iOS Flow 采用更长的静音排空（350 ms）、固定 150 ms 尾音保留、增强末块 ASR 恢复与 partial 兜底，降低弱尾音从结果中消失的概率。
+- **Flow batch ASR fallback**: when pipelined chunk output is clearly shorter than the live partial, the host re-transcribes the full utterance PCM captured during recording (Mac-style safety net). / **Flow 整句 ASR 兜底**：流水线拼接结果明显短于实时 partial 时，主 App 对录音期间累积的整段 PCM 重新识别（对齐 Mac 双保险）。
+
+### Changed
+- **Mac MLX tail drain**: streaming capture now uses the shared `FlowUtteranceEndCoordinator` (silence drain + post-roll) instead of an inline poll loop. / **Mac MLX 尾音排空**：流式采集改用 Shared 层 `FlowUtteranceEndCoordinator`（静音排空 + post-roll），替代内联轮询循环。
 
 ### Added
 - **Mac MLX streaming ASR**: local dictation uses Qwen3-ASR via mlx-audio-swift with overlay partial preview, tail drain, vocabulary prompt, and polish-before-insert. / **Mac MLX 流式 ASR**：本地听写改用 mlx-audio-swift 的 Qwen3-ASR，支持浮层 partial 预览、尾部截断、词库 prompt 与润色后再插入。