Append single char + space as one 2-byte call in _AppendRuns

Add a char-specific overload of _AppendRuns (more specialized than the
generic const T& overload by partial ordering) that flushes any pending
literal buffer, then appends the char value and its trailing space as a
single rs_code.append(pair, 2) call instead of two separate += operations.

Agent-Logs-Url: https://github.com/Cpp2Rust/cpp2rust/sessions/c9bd1adc-01f6-415e-a50c-9e8a4cb1e414

Co-authored-by: nunoplopes <2998477+nunoplopes@users.noreply.github.com>

Author: Copilot

cpp2rust/converter/converter.h

@@ -361,6 +361,18 @@ class Converter : public clang::RecursiveASTVisitor<Converter> {
     _AppendRuns(rs_code, std::array<char, 0>{}, rest...);
   }
 
+  // Head is a single char: flush the pending buffer (if any), then append
+  // the char and its trailing space together as a single 2-byte append.
+  template <std::size_t BufSize, typename... Rest>
+  static void _AppendRuns(std::string &rs_code, std::array<char, BufSize> buf,
+                          char val, const Rest &...rest) {
+    if constexpr (BufSize > 0)
+      rs_code.append(buf.data(), BufSize);
+    const char pair[2] = {val, ' '};
+    rs_code.append(pair, 2);
+    _AppendRuns(rs_code, std::array<char, 0>{}, rest...);
+  }
+
   class Buffer {
     std::string partial_code;
     std::string *full_code;