fix(engine): 辅助码串上的 Ctrl+方向与 Ctrl+Backspace 按单元编辑 (#627)

双拼串里带句中辅助码（反引号段 / 大写段）或开着直接辅助码时，segment_raw_boundaries
直接返回空，Server 只能退回逐字符移动和删除。

现在按请求带着的辅助码段布局切单元：每个音节和挂在它后面的辅码段各算一个单元，
Ctrl+Backspace 可以只删辅码，Ctrl+方向也能停在音节与辅码之间。布局对不上原串时仍
不给边界、退回逐字符。直接辅助码没解析出辅码时请求原样放行，沿用普通双拼切分。

光标前缀重算与按段编辑共用同一套边界，带辅助码的串也随之按光标前缀出候选。

engine/tests/src/test_direct_helpcode.cpp

@@ -258,6 +258,9 @@ void run_session_tests()
                     on->candidates().size() == off->candidates().size(),
                 "Plain shuangpin changed when direct helpcode was turned on.");
         require(on->get_pinyin_sequence() == "uiui", "Plain shuangpin was rewritten by the resolver.");
+        require(on->segment_raw_boundaries() == std::vector<std::size_t>({0, 2, 4}) &&
+                    on->segment_raw_boundaries() == off->segment_raw_boundaries(),
+                "Plain shuangpin lost its unit boundaries when direct helpcode was turned on.");
     }
 
     {
@@ -272,6 +275,9 @@ void run_session_tests()
         require(session->get_pinyin_sequence() == "ui'ui", "The resolver did not strip the helpcode letters.");
         require(session->get_pinyin_segmentation_with_cases() == "uia'uiq",
                 "The helpcode letters were not shown after their syllables.");
+        // 按段删除 / 跳光标的单元：每个音节和它后面的辅码各算一个。
+        require(session->segment_raw_boundaries() == std::vector<std::size_t>({0, 2, 3, 5, 6}),
+                "The direct helpcode letters were not units of their own.");
     }
 
     {
@@ -311,6 +317,8 @@ void run_session_tests()
         require(first_word(*session) == "石狮", "uiab/ui did not keep 石 on the first syllable.");
         require(session->get_pinyin_segmentation_with_cases() == "uiab/'ui",
                 "The four-key code and its slash were not shown after their syllable.");
+        require(session->segment_raw_boundaries() == std::vector<std::size_t>({0, 2, 5, 7}),
+                "The four-key code and its slash were not one unit.");
     }
 
     {