#14806 Cell search tree: Group cells in leaves and store leaf ids in flat arrays

Building the cell search tree was dominated by one small std::vector<int>
allocation per leaf, and a tree with one cell per leaf becomes very deep for
large grids.

- BoundingBoxTree: Store leaf ids in flat arrays, and move the input data
  instead of copying it. The tree shape is unchanged. Queries no longer copy
  the ids of each leaf.
- RigMainGrid: Always group at least 8 cells along K in each leaf.
  findIntersectingCells() removes cells not intersecting the query bounding
  box, so the result is the same set of cells as for a tree with one cell per
  leaf. This also applies to very large grids, which previously got the
  cells of the whole leaf.
- Include cells of nested LGRs and LGR cells in invalid main grid cells in
  the grouped tree, as the previous tree with one cell per leaf did.

ApplicationLibCode/UnitTests/RigMainGridSearchTree-Test.cpp

@@ -0,0 +1,95 @@
+/////////////////////////////////////////////////////////////////////////////////
+//
+//  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 "gtest/gtest.h"
+
+#include "RiaTestDataDirectory.h"
+
+#include "RifReaderOpmCommon.h"
+
+#include "RigEclipseCaseData.h"
+#include "RigMainGrid.h"
+
+#include <QDir>
+
+#include <algorithm>
+
+//--------------------------------------------------------------------------------------------------
+/// The search tree stores several cells in each leaf. Verify that the result is the same as when testing the bounding
+/// box of every valid cell, including LGR cells.
+//--------------------------------------------------------------------------------------------------
+TEST( RigMainGridSearchTree, FindIntersectingCellsMatchesAllCellsTest )
+{
+    QDir    baseFolder( TEST_MODEL_DIR );
+    QString filename = baseFolder.absoluteFilePath( "TEST10K_FLT_LGR_NNC/TEST10K_FLT_LGR_NNC.EGRID" );
+    ASSERT_TRUE( QFile::exists( filename ) );
+
+    cvf::ref<RigEclipseCaseData> caseData = new RigEclipseCaseData( nullptr );
+    cvf::ref<RifReaderOpmCommon> reader   = new RifReaderOpmCommon;
+    ASSERT_TRUE( reader->open( filename, caseData.p() ) );
+
+    RigMainGrid* mainGrid = caseData->mainGrid();
+    mainGrid->computeCachedData();
+    ASSERT_GT( mainGrid->gridCount(), 1u );
+
+    std::vector<cvf::BoundingBox> cellBoundingBoxes( mainGrid->totalCellCount() );
+    for ( size_t cellIdx = 0; cellIdx < mainGrid->totalCellCount(); cellIdx++ )
+    {
+        if ( mainGrid->cell( cellIdx ).isInvalid() ) continue;
+
+        for ( size_t nodeIdx : mainGrid->cell( cellIdx ).cornerIndices() )
+        {
+            cellBoundingBoxes[cellIdx].add( mainGrid->nodes()[nodeIdx] );
+        }
+    }
+
+    const std::vector<double> halfSizes         = { 0.1, 20.0, 200.0 };
+    const size_t              queryCountPerGrid = 20;
+
+    // Query around cells in the main grid and in each LGR
+    size_t queryCount = 0;
+    for ( size_t gridIdx = 0; gridIdx < mainGrid->gridCount(); gridIdx++ )
+    {
+        const RigGridBase* grid   = mainGrid->gridByIndex( gridIdx );
+        const size_t       stride = std::max( size_t( 1 ), grid->cellCount() / queryCountPerGrid );
+
+        for ( size_t localIdx = 0; localIdx < grid->cellCount(); localIdx += stride )
+        {
+            const size_t cellIdx = grid->reservoirCellIndex( localIdx );
+            if ( mainGrid->cell( cellIdx ).isInvalid() ) continue;
+
+            const cvf::Vec3d center   = mainGrid->cell( cellIdx ).center();
+            const double     halfSize = halfSizes[queryCount++ % halfSizes.size()];
+
+            cvf::BoundingBox queryBB( center - cvf::Vec3d( halfSize, halfSize, halfSize ), center + cvf::Vec3d( halfSize, halfSize, halfSize ) );
+
+            std::vector<size_t> expected;
+            for ( size_t i = 0; i < cellBoundingBoxes.size(); i++ )
+            {
+                if ( cellBoundingBoxes[i].isValid() && cellBoundingBoxes[i].intersects( queryBB ) ) expected.push_back( i );
+            }
+
+            std::vector<size_t> found = mainGrid->findIntersectingCells( queryBB );
+            std::sort( found.begin(), found.end() );
+
+            EXPECT_EQ( expected, found ) << "Query around cell " << cellIdx << " in grid " << gridIdx;
+        }
+    }
+
+    EXPECT_GT( queryCount, 2 * queryCountPerGrid );
+}