build: no -Wrestrict warning under gcc 12 (from dev e727ce1d248d)

extended_latin_symbol_pattern() assigned a string literal to a std::string
inside its loop, for which gcc 12 reports a bogus memcpy overlap at -O3.  The
separator is a const char* now.  No behaviour change.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

mac/character.cpp

@@ -133,7 +133,10 @@ const std::regex unresolved_nonascii_re(R"(\^[A-Za-z0-9]{1,6}\^)");
 std::string extended_latin_symbol_pattern()
 {
     std::string result {};
-    std::string sep = "";
+    // A pointer, not a std::string: gcc 12 (the container's compiler) warns
+    // -Wrestrict on assigning a string literal to a std::string here -- a
+    // false positive of that release -- and the separator needs no storage.
+    const char* sep = "";
     for (const auto& nr : extended_latin_symbols) {
         result += sep;
         result += nr;