Port shared-partner changestats to the C++ term API

[Copilot]

The shared-partner changestats were implemented as a dense C macro expansion over edgewise/dyadwise, six SP configurations, and three calculation modes. This change moves the public changestat entry points onto the C++ term API and updates the triadic proposal code to use the same shared-partner dispatch helpers.

• C++ changestat entry points

  • Replaces src/changestats_dgw_sp.c with src/changestats_dgw_sp.cpp.
  • Ports the ddsp, dgwdsp, desp, dgwesp, dnsp, and dgwnsp entry points (plus distribution/bipartite wrappers) to C_CHANGESTAT_CPP(...).
  • Keeps the existing SP traversal semantics intact while expressing the top-level dispatch and stat updates in C++.

• Shared helper dispatch

  • Adds C++ helper functions in src/changestats_dgw_sp.h to dispatch dsp* / esp* logic by L2Type.
  • Consolidates repeated per-type selection in one place, including the nonzero-dyad delta used by proposal code.

• Triadic proposal update

  • Updates src/MHproposals_triadic.cpp to call the new shared-partner helper instead of expanding the legacy header macros directly.
  • Preserves the existing unsupported-RTP behavior in the proposal path.

Example of the new dispatch style:

C_CHANGESTAT_CPP(dgwdsp, {
    auto *spcache = get_spcache(mt);
    L2Type type = static_cast<L2Type>(mt.iinput[0]);

    double cumchange = dsp_path_multiplier(type) == 1
      ? dsp_gw_change<1>(type, tail, head, nwp, edgestate, spcache, alpha, loneexpa)
      : dsp_gw_change<2>(type, tail, head, nwp, edgestate, spcache, alpha, loneexpa);

    mt.stat[0] = edgestate ? -cumchange : cumchange;
  })

[krivit]

@copilot Also modify the sp_change() functions to use the C++ network API and incorporate them as templates.

[Copilot]

@copilot Also modify the sp_change() functions to use the C++ network API and incorporate them as templates.

Addressed in a9a9b21.

[krivit]

@copilot Assume that changestats_dgw_sp.h will only ever be invoked from C++, and remove no-longer-used macros.

[Copilot]

@copilot Assume that changestats_dgw_sp.h will only ever be invoked from C++, and remove no-longer-used macros.

Addressed in 122588c.