Null-area face aborts the run on valid corner-point pinch-outs (regression from #3977)

Description

centroid_3DPolygon() in ComputationalGeometry.hpp calls GEOS_ERROR when a polygon has zero area. This aborts the run on corner-point meshes that contain pinch-outs. A pinch-out is ordinary reservoir geology, not a defect in the mesh.

Regression

Before #3977 (01a4f15b2e, "Change to local Newell's normal") the branch returned zero:

if( area > areaTolerance )       { /* normalize */ }
else if( area < -areaTolerance ) { GEOS_ERROR( "Negative area found" ); }
else                             { return 0.0; }

#3977 replaced that return 0.0 with GEOS_ERROR( "Null area found" ). The return 0.0 looks deliberate: a zero-area face gives zero transmissibility, which is the physically correct outcome, because no flow crosses a face of zero area. #3977 was about Newell normals, so the change to the tolerance branch appears incidental.

Concrete case

A corner-point mesh of 1761 hexahedra, converted from a standard Eclipse-style deck. Eight consecutive cells each have one vertical edge of zero length, while the other three measure 3.9 to 4.7 m. One lateral face of each cell collapses to a triangle.

The cells are valid and active: non-zero volume, and porosity between 0.05 and 0.14. The mesh holds 8 degenerate faces among several thousand.

On current develop this mesh cannot be run at all. The first degenerate face aborts initialization.

Suggested fix

Change GEOS_ERROR to GEOS_WARNING in the null-area branch and keep return 0.0. This restores the pre-#3977 behavior and keeps the diagnostic that #3977 added. The negative-area branch should stay a GEOS_ERROR, because a negative area does signal a real defect.

A stricter alternative would be an input flag that lets the user choose between abort and warn, with warn as the default for corner-point meshes.

Impact

With the warning, two such decks run to completion and reproduce our reference results bit for bit over 500 simulated years. Each reports 16 warnings, from the 8 degenerate faces.

Environment

GEOS develop at 24a8410b51, gcc 13.3.0, VTK 9.4.2, serial run.