Fix igraph_t ownership issues in NetlistGraph

[julianspeith]

Two related memory-safety defects around NetlistGraph's igraph_t member.

1. Destroying an uninitialized igraph_t.

   NetlistGraph(Netlist*) initializes only m_nl, deliberately leaving m_graph to be filled in afterwards by the factory that called it (igraph_create() in from_netlist(), igraph_empty() in from_netlist_no_edges(), igraph_copy() in copy()). The destructor, however, called igraph_destroy(&m_graph) unconditionally.

   When a factory bails out between construction and that initialization, the std::unique_ptr holding the half-constructed graph unwinds and igraph_destroy() runs over uninitialized memory. Reachable paths:

   • from_netlist(): constructed at the top, returns early if igraph_vector_int_init() for the edge vector fails, well before igraph_create() is reached.
   • copy(): constructed at the top, returns early if igraph_copy() itself fails.

   Both require an igraph allocation to fail, so this does not show up in
   normal runs, but it is a genuine crash path rather than a leak.

   Fixed by tracking whether m_graph was actually initialized and having
   the destructor honour that. The flag is set only after the respective
   igraph call has reported success. m_graph_ptr is now default-initialized
   to nullptr as well, since the same constructor left it dangling.

2. NetlistGraph was implicitly copyable.

   igraph_t is a plain C struct holding heap pointers, so the implicitly generated copy constructor and copy assignment operator bitwise-copied m_graph and left two NetlistGraph objects aliasing the same graph internals; the destructor would then igraph_destroy() them twice. The copy's m_graph_ptr would additionally point into the source object.

   Nothing copies a NetlistGraph today -- every factory hands out a std::unique_ptr, the python bindings hold it via RawPtrWrapper and pass it by const reference, and callers wanting a duplicate use the explicit copy() method -- so this was latent. Deleting the copy operations enforces that invariant at compile time instead of leaving it to convention.

Verified by syntax-checking all 14 translation units under plugins/graph_algorithm and plugins/hawkeye (including both python binding files) against the modified header; all compile unchanged. Both files are clang-format clean.