Solver backends

Every problem class exposes a solver parameter on get_solution(). Two backends are supported:

solver value	Backend	Notes
"highs" (default)	HiGHS via highspy	Always available; cut-based formulation solved iteratively (re-solve loop).
"gurobi"	Gurobi via gurobipy	Optional; requires gurobipy and a valid Gurobi license. Connectivity cuts are injected as lazy constraints inside a branch-and-cut callback, which lets Gurobi exploit its full branch-and-bound tree.

# Use HiGHS (default — no extra installation required)
solution = SteinerProblem(graph, terminal_groups).get_solution()

# Use Gurobi (requires gurobipy + license)
solution = SteinerProblem(graph, terminal_groups).get_solution(solver="gurobi")

Both solvers implement the same cut-based (DO-D) formulation from Markhorst et al. (2025) and produce identical optimal solutions. Gurobi may be faster on larger instances because callbacks avoid repeated re-solves from scratch.