refactor: split core/ui/apps targets and add unit tests

Move all business logic into liblivecaption_core (no Qt dependency) and
the widgets into liblivecaption_ui, leaving two executables: the caption
window and the CLI, which no longer links Qt Widgets. Abstract the ASR
backend behind IAsr so the streaming policy and the live pipeline can be
tested with a scripted fake backend instead of a real model.

把全部业务逻辑下沉到不依赖 Qt 的 liblivecaption_core，界面独立成
liblivecaption_ui，拆出悬浮窗与命令行两个可执行程序（CLI 不再链接 Qt
Widgets）；并用 IAsr 抽象推理后端，使流式策略与实时管线可脱离真模型
用假实现做单元测试。测试还抓到一个真 bug：段恰好结束在 chunk 边界时
只提交了稳定部分，最后几个词被丢掉，已修。

tests/tst_transcript.cpp

@@ -0,0 +1,48 @@
+#include <QtTest>
+
+#include "transcript.h"
+
+// 输出解析：Qwen3-ASR 的原始输出形如 "language English<asr_text>正文"
+class TstTranscript : public QObject {
+    Q_OBJECT
+
+private slots:
+    void splitsLanguageAndText()
+    {
+        const auto r = lc::parseAsrOutput("language English<asr_text>He hoped there would be stew.");
+        QCOMPARE(QString::fromStdString(r.language), QString("English"));
+        QCOMPARE(QString::fromStdString(r.text), QString("He hoped there would be stew."));
+    }
+
+    void handlesChinese()
+    {
+        const auto r = lc::parseAsrOutput("language Chinese<asr_text>不排除购房者可以在互联网上下单。");
+        QCOMPARE(QString::fromStdString(r.language), QString("Chinese"));
+        QCOMPARE(QString::fromStdString(r.text), QString("不排除购房者可以在互联网上下单。"));
+    }
+
+    void withoutMarkerKeepsWholeText()
+    {
+        const auto r = lc::parseAsrOutput("  just text  ");
+        QVERIFY(r.language.empty());
+        QCOMPARE(QString::fromStdString(r.text), QString("just text"));
+    }
+
+    void trimsSurroundingWhitespaceOfBody()
+    {
+        const auto r = lc::parseAsrOutput("language English<asr_text>  hi there  ");
+        QCOMPARE(QString::fromStdString(r.text), QString("hi there"));
+    }
+
+    void displayNameMapping()
+    {
+        QCOMPARE(QString::fromStdString(lc::languageDisplayName("English")), QString("英语"));
+        QCOMPARE(QString::fromStdString(lc::languageDisplayName("Chinese")), QString("中文"));
+        // 未收录的语言原样返回
+        QCOMPARE(QString::fromStdString(lc::languageDisplayName("Klingon")), QString("Klingon"));
+        QVERIFY(lc::languageDisplayName("").empty());
+    }
+};
+
+QTEST_MAIN(TstTranscript)
+#include "tst_transcript.moc"