Add copying utility methods to Arena

style.md

@@ -396,6 +396,27 @@ operations = {};  // Default construct - clear() won't work correctly
 arena.reset();  // Reset arena memory
 ```
 
+### Arena String Copying
+
+- **Always use `Arena::copy_string()`** for copying string data into arena memory
+- **Avoid manual allocation and memcpy** for string copying
+- **Use `Arena::allocate_span<T>()`** for array allocations instead of manual span construction
+
+```cpp
+// Preferred - unified arena methods
+std::string_view copy = arena.copy_string(original_string);
+auto buffer = arena.allocate_span<char>(1024);
+auto strings = arena.allocate_span<std::string_view>(count);
+
+// Avoid - manual allocation and copying
+char *copied = arena.allocate<char>(str.size());
+std::memcpy(copied, str.data(), str.size());
+std::string_view copy(copied, str.size());
+
+// Avoid - manual span construction
+auto span = std::span{arena.allocate<std::string_view>(count), count};
+```
+
 ### Resource Management
 
 - **RAII** everywhere - constructors acquire, destructors release