Part 2: Operator Snapshot Layer – per-substep DW state exposure

[wiesnerfriedman]

• Add openswmm_operator_snapshot.h: C API for snapshot callback and poll-mode
• Add OperatorSnapshotState.hpp: per-instance snapshot storage and staging buffers
• Wire snapshot populate + callback into SWMMEngine::stepRouting()
• Add DWSolver::populateSnapshot() with zero-copy pointers into solver buffers
• Wire iteration history (resetIterCount/recordResidual) into Picard loop
• Add C API: swmm_set_operator_snapshot_callback, swmm_get_operator_snapshot, swmm_enable_iteration_history, swmm_get_iteration_residual
• Fix error code redefinition conflict (SWMM_ERR_WRONG_STATE alias)
• Add IntelliSense workaround for dllimport false positives
• Add test_operator_snapshot.cpp (4 tests) and bench_operator_snapshot.cpp
• Register test and benchmark in CMakeLists.txt