Eliminate double-lookups in map/set operations

[Copilot]

Containment checks followed by insertion or access into the same container incur two hash lookups where one suffices. Three such patterns are addressed:

• AddRuleForUserDefinedType (mapper.cpp): types_.contains + types_[key] = replaced with a single try_emplace:

  // Before: two lookups
  if (types_.contains(cpp_name)) { return; }
  types_[cpp_name] = TranslationRule::TypeTgt::Plain(rs_name);
  
  // After: one lookup
  if (!types_.try_emplace(cpp_name, TranslationRule::TypeTgt::Plain(rs_name)).second) {
      return;
  }

• matchTemplate (mapper.cpp): captured.find + captured[name] = collapsed into a single try_emplace; the returned iterator is used directly to assign the captured value in the else branch, eliminating the second lookup on the insert path.

• visit_token (syntactic.rs): token.text().to_string() was called twice — once for generic_names.contains() and again to populate BodyFragment::Generic. The result is now computed once and reused.