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。

tests/tst_streamingasr.cpp

@@ -150,6 +150,33 @@ private slots:
         QVERIFY2(all.contains("hello there world"), qPrintable(all));
     }
 
+    // 追加式：每块音频作为新一轮追加，文本立即定稿，不回喂任何前缀
+    void appendModeCommitsEachChunkImmediately()
+    {
+        FakeAsr fake;
+        fake.scripted = {"language English<asr_text>one two",
+                         " language English<asr_text> three four"};
+
+        lc::StreamOptions o = baseOptions();
+        o.appendOnly = true;
+
+        lc::StreamingAsr stream(fake, o);
+        std::vector<lc::StreamingAsr::Step> steps;
+        const auto cb = [&steps](const lc::StreamingAsr::Step & s) { steps.push_back(s); };
+
+        const auto c = chunk(16000);
+        stream.feed(c.data(), c.size(), cb);
+        stream.feed(c.data(), c.size(), cb);
+
+        QCOMPARE(fake.turnsSeen.size(), size_t(2));
+        QVERIFY2(fake.prefixesSeen.empty(), "追加式不应该回喂前缀");
+
+        QCOMPARE(steps.size(), size_t(2));
+        QCOMPARE(q(steps[0].committed), QString("one two"));
+        QVERIFY(steps[0].tail.empty());                  // 追加式没有"不稳定尾部"
+        QVERIFY(q(steps[1].committed).contains("three four"));
+    }
+
     // 同一次 feed 里跨多块也要正确切分
     void splitsMultipleChunksInOneFeed()
     {