Remove duplicate getEntityNDIndex definition and fix entity indexing bugs

[Copilot]

The TODO in DOFHandler.hpp was misleading—getEntityNDIndex was already implemented inline in DOFHandler.h. However, inspection revealed critical bugs in the helper functions it dispatches to.

Changes

• Removed duplicate definition: Eliminated out-of-line declaration with TODO in DOFHandler.hpp (lines 296-300)

• Fixed vertex/edge indexing: Replaced incorrect bit operations with explicit offset arrays matching counter-clockwise ordering from fillLagrangeDOFMappingTable

  • getVertexNDIndex: 2D vertex 2 was computing [0,1] instead of [1,1]
  • getEdgeNDIndex: All 3D edge types (EdgeX, EdgeY, EdgeZ) produced wrong orderings

• Completed getFaceNDIndex: Added missing FaceXZ and FaceYZ implementations for 3D

• Fixed spelling: Hexahedron → Hexaedron to match Entity.h definition

// Before: incorrect bit operations
vertexNDIndex[0] += vertexLocalIndex & 1;        // 0,1,0,1 ❌
vertexNDIndex[1] += (vertexLocalIndex >> 1) & 1; // 0,0,1,1 ❌

// After: explicit counter-clockwise ordering
constexpr size_t offsets[4][2] = {{0, 0}, {1, 0}, {1, 1}, {0, 1}};
vertexNDIndex[0] += offsets[vertexLocalIndex][0]; // 0,1,1,0 ✓
vertexNDIndex[1] += offsets[vertexLocalIndex][1]; // 0,0,1,1 ✓

The indexing bugs would have caused incorrect DOF mappings throughout FEM assembly operations.

Original prompt

Work on TODO: implement the mapping to get entity NDIndex (from src/FEM/DOFHandler.hpp)

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