Ownership & move-only Graph

[allnes]

Please make Graph move-only and avoid sharing node ownership. Copying a graph is ambiguous and expensive; move-only makes lifetime explicit and prevents accidental deep/shallow copies. Also, keep a single owner of layers inside Graph and pass raw pointers (or ids) for wiring to avoid refcount overhead on the hot path.

// graph.hpp
class Graph {
public:
  Graph(const Graph&) = delete;
  Graph& operator=(const Graph&) = delete;
  Graph(Graph&&) noexcept = default;
  Graph& operator=(Graph&&) noexcept = default;
  ~Graph() = default;
  // ...
};

// preferred API inside Graph
Layer* addLayer(std::unique_ptr<Layer> L); // returns non-owning handle for connections

It's avoids atomic refcount churn from shared_ptr, eliminates hidden cycles, and makes lifetime rules crystal clear.

[aobolensk]

@allnes @Semyon1104 can this be closed?