SolveSpace's solid-model operations (Boolean ops, STEP export) are handled by a custom NURBS/BRep kernel. This PR wires in OCCT as an opt-in replacement via find_package, providing more robust Boolean operations, standards-compliant STEP export, and new IGES export capability.

Build system

• cmake/FindOpenCASCADE.cmake — new module; tries OpenCASCADEConfig.cmake first, then pkg-config, then manual header/library search
• CMakeLists.txt — new ENABLE_OPENCASCADE cache option (default OFF); calls find_package(OpenCASCADE REQUIRED) when enabled; sets HAVE_OPENCASCADE for config.h
• src/CMakeLists.txt — conditionally compiles occt_kernel.cpp and links OCCT into solvespace-core

cmake -DENABLE_OPENCASCADE=ON ..   # opt-in; requires OCCT 7.x installed

OCCT integration layer (src/occt_kernel.h/cpp)

• SSurfaceToGeomSurface / SBezierToGeomCurve — converts SolveSpace's single-span rational Bezier patches/curves to Geom_BSplineSurface / Geom_BSplineCurve
• ComputePCurve — samples 3-D trim curves and projects them onto the surface UV domain via ShapeAnalysis_Surface to produce 2-D pcurves for BRepBuilderAPI_MakeFace
• SShellToOCCTShape — full shell conversion: face per surface with trim wires, sewn with BRepBuilderAPI_Sewing, healed with ShapeFix_Shape
• OCCTShapeToSShell — reverse conversion after Boolean ops; handles rational/non-rational B-splines, falls back via GeomConvert
• OCCTExportStepFile / OCCTExportIgesFile — delegates to STEPControl_Writer / IGESControl_Writer
• OCCTBooleanUnion/Difference/Intersection — wraps BRepAlgoAPI_Fuse/Cut/Common

Geometry kernel hooks

• src/srf/boolean.cpp — MakeFromUnionOf/DifferenceOf/IntersectionOf try OCCT first; fall back to built-in kernel if conversion fails (no regression when OCCT is off or conversion is unsupported)

STEP / IGES export

• ExportSurfacesTo() short-circuits to OCCTExportStepFile() when HAVE_OPENCASCADE is defined
• New StepFileWriter::ExportIgesTo() delegates to OCCTExportIgesFile(); errors gracefully when built without OCCT
• File-extension routing in both GUI (solvespace.cpp) and CLI (entrycli.cpp): .iges/.igs → ExportIgesTo, otherwise STEP
• IGES added to SurfaceFileFilters under #ifdef HAVE_OPENCASCADE

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