A6: gdp.cuttingplane modernization #8

gdp.cuttingplane (Sawaya & Grossmann 2005 projection cuts) is effectively linear-only (FME cut path skips any body with polynomial_degree() != 1, cuttingplane.py:965), non-structural (fixed vars treated as data — unfixing invalidates the model), capped at 100 cuts, requires an active objective and a dual-providing QP-capable solver.

Scope:

• Nonlinear separation (project onto relaxed hull with nonlinear constraints — needs NLP separation problem handling).
• Ship a basic-step tighten_relaxation callback (depends on A1).
• Cut management (aging/violation filtering beyond current thresholds).
• Revisit the private-API reach into FME (cuttingplane.py:977 TODO).

Part of roadmap epic #2 (item A6).