feat: 实现 Linux Fcitx5 实时输入链路

加入 Unix Socket v2 协议、会话状态机、动态快捷键、完整 preedit 更新、结束提交以及断线重连和安全取消。

AI-assisted-by: Codex
AI-model: gpt-5.6-sol
AI-reasoning-effort: medium
Co-authored-by: Codex <codex@openai.com>

linux/fcitx5/protocolsession_test.cpp

@@ -0,0 +1,21 @@
+#include "protocolsession.h"
+
+#include <cassert>
+
+int main() {
+    ProtocolSession session;
+    assert(!session.active());
+    session.start("round-1");
+    assert(session.active());
+    assert(session.accept("round-1", 1));
+    assert(!session.accept("round-1", 1));
+    assert(!session.accept("round-1", 0));
+    assert(!session.accept("old-round", 2));
+    assert(session.accept("round-1", 3));
+    session.reset();
+    assert(!session.active());
+    assert(!session.accept("round-1", 4));
+    session.start("round-2");
+    assert(!session.accept("round-1", 100));
+    assert(session.accept("round-2", 1));
+}