refactor: Small style fixups in src/kernel/bitcoinkernel.cpp

* Use type alias TranslateFn:
  https://github.com/bitcoin/bitcoin/pull/30595#discussion_r2653828562
* Use std::span::data:
  https://github.com/bitcoin/bitcoin/pull/30595#discussion_r2653829743
* Use the ref helper:
  https://github.com/bitcoin/bitcoin/pull/30595#discussion_r2653829991
* Reword error handling section:
  https://github.com/bitcoin/bitcoin/pull/30595#discussion_r2653843805

src/kernel/bitcoinkernel.cpp

@@ -41,7 +41,6 @@
 #include <cstring>
 #include <exception>
 #include <functional>
-#include <iterator>
 #include <list>
 #include <memory>
 #include <span>
@@ -56,7 +55,7 @@ using util::ImmediateTaskRunner;
 
 // Define G_TRANSLATION_FUN symbol in libbitcoinkernel library so users of the
 // library aren't required to export this symbol
-extern const std::function<std::string(const char*)> G_TRANSLATION_FUN{nullptr};
+extern const TranslateFn G_TRANSLATION_FUN{nullptr};
 
 static const kernel::Context btck_context_static{};
 
@@ -84,7 +83,7 @@ public:
     //
     void write(std::span<const std::byte> src)
     {
-        if (m_writer(std::data(src), src.size(), m_user_data) != 0) {
+        if (m_writer(src.data(), src.size(), m_user_data) != 0) {
             throw std::runtime_error("Failed to write serialization data");
         }
     }
@@ -113,13 +112,13 @@ struct Handle {
     static C* create(Args&&... args)
     {
         auto cpp_obj{std::make_unique<CPP>(std::forward<Args>(args)...)};
-        return reinterpret_cast<C*>(cpp_obj.release());
+        return ref(cpp_obj.release());
     }
 
     static C* copy(const C* ptr)
     {
         auto cpp_obj{std::make_unique<CPP>(get(ptr))};
-        return reinterpret_cast<C*>(cpp_obj.release());
+        return ref(cpp_obj.release());
     }
 
     static const CPP& get(const C* ptr)

src/kernel/bitcoinkernel.h

@@ -82,9 +82,9 @@ extern "C" {
  * @section error Error handling
  *
  * Functions communicate an error through their return types, usually returning
- * a nullptr, 0, or false if an error is encountered. Additionally, verification
+ * a nullptr or a status code as documented by the returning function.
- * functions, e.g. for scripts, may communicate more detailed error information
+ * Additionally, verification functions, e.g. for scripts, may communicate more
- * through status code out parameters.
+ * detailed error information through status code out parameters.
  *
  * Fine-grained validation information is communicated through the validation
  * interface.