Remove KV serialiser forward declaration indirection

[Copilot]

KV consumers only need a stable serialiser/deserialiser interface, not the concrete raw wrapper templates. The previous serialiser_declare.h indirection duplicated type knowledge and kept kv_types.h coupled to template forward declarations.

• Abstract interfaces

  • Added virtual KvStoreSerialiser and KvStoreDeserialiser interfaces directly in kv_types.h.
  • Kept map/change-set APIs typed against these abstract interfaces.

• Concrete raw implementations

  • Made GenericSerialiseWrapper and GenericDeserialiseWrapper implement the new interfaces.
  • Added RawKvStoreSerialiser / RawKvStoreDeserialiser aliases in kv_serialiser.h for construction sites.

• Removed obsolete declaration layer

  • Deleted serialiser_declare.h.
  • Updated Store, Tx, and Snapshot code to construct the raw concrete wrappers explicitly.

class KvStoreSerialiser
{
public:
  virtual ~KvStoreSerialiser() = default;

  virtual void start_map(const std::string& name, SecurityDomain domain) = 0;
  virtual void serialise_raw(const std::vector<uint8_t>& raw) = 0;
  virtual std::vector<uint8_t> get_raw_data() = 0;
};