#13890 Faults: Add FAULTDIST result for a user-selected subset of faults

The existing FAULTDIST result always considered every fault in the main
grid. Users with many faults need to compute distance fields against a
named subset, so this adds a Fault Distance Results collection under
each view's Faults node. Each entry holds a multiselect of faults and a
name (FAULTDIST1, FAULTDIST2, ...) and publishes the result into the
Generated cell-result category.

The per-cell BVH-based distance loop was extracted from
RigFaultDistanceResultCalculator into a reusable utility that accepts
the subset of faults to include; the original all-faults entry point
delegates to the same utility and keeps the static-native FAULTDIST
behavior unchanged.

ApplicationLibCode/ReservoirDataModel/ResultCalculators/RigFaultDistanceCalculator.cpp

@@ -0,0 +1,134 @@
+/////////////////////////////////////////////////////////////////////////////////
+//
+//  Copyright (C) 2026-     Equinor ASA
+//
+//  ResInsight is free software: you can redistribute it and/or modify
+//  it under the terms of the GNU General Public License as published by
+//  the Free Software Foundation, either version 3 of the License, or
+//  (at your option) any later version.
+//
+//  ResInsight is distributed in the hope that it will be useful, but WITHOUT ANY
+//  WARRANTY; without even the implied warranty of MERCHANTABILITY or
+//  FITNESS FOR A PARTICULAR PURPOSE.
+//
+//  See the GNU General Public License at <http://www.gnu.org/licenses/gpl.html>
+//  for more details.
+//
+/////////////////////////////////////////////////////////////////////////////////
+
+#include "RigFaultDistanceCalculator.h"
+
+#include "RigActiveCellInfo.h"
+#include "RigCell.h"
+#include "RigFault.h"
+#include "RigMainGrid.h"
+
+#include "cvfBoundingBoxTree.h"
+
+#include <limits>
+
+//--------------------------------------------------------------------------------------------------
+///
+//--------------------------------------------------------------------------------------------------
+void RigFaultDistanceCalculator::computeFaultDistances( const RigMainGrid*                  mainGrid,
+                                                        const RigActiveCellInfo*            activeCellInfo,
+                                                        const std::vector<const RigFault*>& faultsToInclude,
+                                                        std::vector<double>&                resultValues )
+{
+    if ( !mainGrid || !activeCellInfo ) return;
+
+    const auto activeCells = activeCellInfo->activeReservoirCellIndices();
+    if ( activeCells.empty() ) return;
+
+    if ( resultValues.size() < activeCells.size() )
+    {
+        resultValues.resize( activeCells.size(), std::numeric_limits<double>::infinity() );
+    }
+
+    // Collect fault face centers from the requested subset of faults.
+    std::vector<cvf::Vec3d> faultFaceCenters;
+    for ( const RigFault* fault : faultsToInclude )
+    {
+        if ( !fault ) continue;
+        for ( const RigFault::FaultFace& faultFace : fault->faultFaces() )
+        {
+            if ( faultFace.m_nativeReservoirCellIndex >= mainGrid->cellCount() ) continue;
+            const RigCell& cell = mainGrid->cell( faultFace.m_nativeReservoirCellIndex );
+            if ( cell.isInvalid() ) continue;
+            faultFaceCenters.push_back( cell.faceCenter( faultFace.m_nativeFace ) );
+        }
+    }
+
+    if ( faultFaceCenters.empty() ) return;
+
+    // Create bounding box tree for all face centers
+    cvf::BoundingBoxTree searchTree;
+    {
+        std::vector<size_t>           faceIndicesForBoundingBoxes;
+        std::vector<cvf::BoundingBox> faceBBs;
+
+        size_t faceCenterIndex = 0;
+        for ( const auto& faultFaceCenter : faultFaceCenters )
+        {
+            cvf::BoundingBox bb;
+            bb.add( faultFaceCenter );
+            faceBBs.push_back( bb );
+            faceIndicesForBoundingBoxes.push_back( faceCenterIndex++ );
+        }
+        searchTree.buildTreeFromBoundingBoxes( faceBBs, &faceIndicesForBoundingBoxes );
+    }
+
+    const auto nodes      = mainGrid->nodes();
+    const auto mainGridBB = mainGrid->boundingBox();
+
+#pragma omp parallel for
+    for ( int activeIndex = 0; activeIndex < static_cast<int>( activeCells.size() ); activeIndex++ )
+    {
+        auto cellIdx = activeCells[activeIndex];
+        if ( cellIdx.value() == cvf::UNDEFINED_SIZE_T ) continue;
+
+        const RigCell& cell = mainGrid->cell( cellIdx.value() );
+        if ( cell.isInvalid() ) continue;
+
+        std::vector<size_t> candidateFaceIndices;
+        {
+            cvf::BoundingBox bb;
+            const auto&      cellIndices = cell.cornerIndices();
+            for ( const auto& i : cellIndices )
+            {
+                bb.add( nodes[i] );
+            }
+
+            searchTree.findIntersections( bb, &candidateFaceIndices );
+
+            bool bbIsBelowThreshold = true;
+            while ( candidateFaceIndices.empty() && bbIsBelowThreshold )
+            {
+                if ( bb.extent().x() > mainGridBB.extent().x() * 2 )
+                {
+                    bbIsBelowThreshold = false;
+                    break;
+                }
+                if ( bb.extent().y() > mainGridBB.extent().y() * 2 )
+                {
+                    bbIsBelowThreshold = false;
+                    break;
+                }
+
+                bb.expand( bb.extent().x() );
+                searchTree.findIntersections( bb, &candidateFaceIndices );
+            }
+        }
+
+        // Find closest fault face
+        double shortestDistance = std::numeric_limits<double>::infinity();
+
+        for ( const auto& faultFaceIndex : candidateFaceIndices )
+        {
+            const cvf::Vec3d& faultFaceCenter = faultFaceCenters[faultFaceIndex];
+            shortestDistance                  = std::min( cell.center().pointDistance( faultFaceCenter ), shortestDistance );
+        }
+
+        resultValues[activeIndex] = shortestDistance;
+    }
+}