Shape Healing - Guard ShapeFix_Face::Perform against non-face context replacement

[gsdali]

Fixes #1322.

Problem

ShapeFix_Face::Perform casts Context()->Apply(myFace) to TopoDS_Face via TopoDS::Face(S.EmptyCopied()) in three places, none of which check the shape type:

TopoDS_Shape S = myFace;
if (!Context().IsNull())
  S = Context()->Apply(myFace);
TopoDS_Shape emptyCopied = S.EmptyCopied();
TopoDS_Face  tmpFace     = TopoDS::Face(emptyCopied);   // unchecked

When a fix sharing the same ShapeBuild_ReShape context has already replaced the face with a compound (e.g. a self-intersecting face split into several faces), Apply() returns a non-face. The unchecked cast then builds an invalid TopoDS_Face handle over a compound TShape; a subsequent operation corrupts the shape and the process aborts with an uncatchable OS signal:

BRep_TEdge::EmptyCopy() const
BRep_Tool::Curve(TopoDS_Edge, ...)
ShapeAnalysis_Edge::Curve3d(...)
ShapeFix_Face::FixOrientation(...)
ShapeFix_Face::Perform(...)

The fault address varies run to run (SIGSEGV / SIGBUS) - a heap-corruption signature. Diagnosed by linking a debug-built ShapeFix_Face.cxx: at -O0 the cast throws Standard_TypeMismatch at the first site with Apply(myFace).ShapeType() == TopAbs_COMPOUND while myFace is still a TopAbs_FACE.

Fix

The compound replacement already exists on entry to Perform (it was recorded by a prior face's fix), so a single guard at the top of Perform covers all three cast sites: if Context()->Apply(myFace) is not a face, record it as the result and return - the replacement is already in the context, so there is nothing to fix here.

Reproducer / test

Extrude a 4-point "bowtie" face (self-intersecting wire) into a prism and run ShapeFix_Shape:

BRepBuilderAPI_MakePolygon mp;
mp.Add(gp_Pnt(0,0,0)); mp.Add(gp_Pnt(1,1,0)); mp.Add(gp_Pnt(1,0,0)); mp.Add(gp_Pnt(0,1,0)); mp.Close();
BRepBuilderAPI_MakeFace mf(mp.Wire(), Standard_True);
BRepPrimAPI_MakePrism prism(mf.Face(), gp_Vec(0,0,1)); prism.Build();
Handle(ShapeFix_Shape) sfs = new ShapeFix_Shape(prism.Shape()); sfs->Perform();  // crash on master

Crashes on current master; fixed here. Added GTest ShapeFix_ShapeTest.HealPrismFromSelfIntersectingFace.

Validation

Verified by compiling the patched ShapeFix_Face.cxx and linking it over the library: the bowtie reproducer and four real mesh-derived fixtures survive (5/5), and the prism now heals to a valid 8-face solid. ShapeFix_Shape output on valid box/sphere/cylinder is unchanged (the guard only triggers for a non-face replacement, which never happens for a well-formed face).

Reported downstream and isolated at SecondMouseAU/OCCTSwift#263.

Draft pending CLA confirmation and CI.