feat(stream): add append-only streaming mode with flat cost

Add an append-only streaming path that feeds each audio chunk as a new
conversation turn, so nothing is ever recomputed: the KV grows by
append, no prefix is re-decoded and no audio is re-encoded. Per-step
cost becomes flat instead of growing with the segment (RTF 1.06 -> 0.26
on 62.6s of continuous speech), but accuracy requires ~8s chunks because
the audio encoder needs its 8s attention window, so text only appears
every 8s. The incremental mode stays the default.

新增追加式流式：每块音频作为一个新的对话轮追加，前缀不重算、旧音频不重
编码，单步成本不再随段落增长（连续 62.6s 语音 RTF 从 1.06 降到 0.26）。
但准确率要求 chunk≈8s，否则会被截断，所以出字粒度是 8s；增量式仍为默认。

apps/common/appcli.h

@@ -44,7 +44,8 @@ inline void addStreamOptions(QCommandLineParser & p)
     p.addOption(makeOption("stream-segment", "单段最长时长，单位秒；0=不限", "sec", "30"));
     p.addOption(makeOption("unfixed-tokens", "前缀回滚丢弃的 token 数", "n", "5"));
     p.addOption(makeOption("unfixed-chunks", "冷启动不带前缀的 chunk 数", "n", "2"));
-    p.addOption(makeOption("stream-mode", "流式后端: incremental 或 window", "mode", "incremental"));
+    p.addOption(makeOption("stream-mode", "流式后端: incremental / append / window", "mode",
+                           "incremental"));
 }
 
 inline AsrOptions asrOptions(const QCommandLineParser & p)
@@ -67,7 +68,8 @@ inline StreamOptions streamOptions(const QCommandLineParser & p)
     o.unfixedTokenNum   = p.value("unfixed-tokens").toInt();
     o.unfixedChunkNum   = p.value("unfixed-chunks").toInt();
     o.maxTokensPerStep  = p.value("max-tokens").toInt();
-    o.incremental       = p.value("stream-mode") != QLatin1String("window");
+    o.incremental       = p.value("stream-mode") == QLatin1String("incremental");
+    o.appendOnly        = p.value("stream-mode") == QLatin1String("append");
     return o;
 }