Make Solver Interface Implementation Plan

[dwr-zroy]

Objective

Create a step by step plan for implementing a common Solver interface, accounting for #214.

[dwr-hzamanisabzi]

Solver Interface Implementation Plan:

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The WRIMS engine supports multiple linear-programming solvers, CBC, Gurobi, and XA, but each one is wired into the system through its own dedicated code path. In ControllerBatch, a long chain of if/else blocks inspects the solver name, calls solver-specific static methods, and then writes results back through solver-specific output routines. This means every time we add a solver, fix a bug, or change how results are logged, we have to touch multiple parallel code paths that were never designed to share a common contract.

The goal of this plan is to finish what we already started in Phase 1: we already have first version of a clean Solver interface and adapter classes sitting on a feature branch, but they are not yet plugged into the engine. The implementation strategy is deliberately incremental and low-risk. We introduce a feature flag so the existing legacy paths remain the default and are completely untouched. Behind that flag, a new runModelUnified() method replaces the if/else dispatch with a single polymorphic Solver.solve() call, the factory picks the right adapter, the adapter delegates to the underlying solver library, and a new bridging layer maps the unified SolverResult back into the legacy data structures that the rest of the engine expects. We start with CBC because it is the default solver and gives us the broadest test coverage, then layer in XA, which already has full sensitivity-analysis support in its adapter, followed by GUROBI. Once all three solvers produce identical results through the unified path, we enable controlling-goals output powered by the GoalAnalyzer that is already built, and only then do we begin deprecating the old code paths. Every step is gated by comparison tests that run the same study through both the legacy and unified paths and verify numerical agreement to within a tight tolerance, so we never ship a change that could silently alter model results.

Step by step Solver implementation plan has been described in the following confluence page:
https://dwrmso.atlassian.net/wiki/spaces/WRIMS/pages/758710274/Solver+Interface+Implementation+Plan

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