fix(candwnd): measure and size candidate wrap at 600 DIP

Force wrap-aware measure, serialize FineTune after measure content
updates, and preserve flip-up host height via MarginTop.

feat: lazily load single-letter Pinyin candidates

- query the top 24 single-letter candidates from the database
- load the complete candidate list on the first page-down action
- apply the same behavior to Shuangpin and Quanpin
- resolve Shuangpin initials according to the active profile
- preserve candidate frequency adjustment and pinning
- disable deletion for single-letter candidates
- add regression tests for Shuangpin v-to-zh mapping and lazy expansion

server/src/session/engine_input_session.cpp

@@ -169,6 +169,11 @@ const std::vector<IInputSession::WordItem> &EngineInputSession::get_candidates()
     return session_.get_candidates();
 }
 
+bool EngineInputSession::expand_initial_candidates()
+{
+    return session_.expand_initial_candidates();
+}
+
 const QueryRequest &EngineInputSession::request() const
 {
     return session_.get_request();
@@ -276,6 +281,10 @@ int EngineInputSession::pin_candidate(std::string pinyin, std::string word)
 
 int EngineInputSession::remove_candidate(std::string pinyin, std::string word)
 {
+    if (!is_wubi() && remove_delimiters(request().raw_input).size() == 1)
+    {
+        return -1;
+    }
     return session_.delete_by_pinyin_and_word(std::move(pinyin), std::move(word));
 }