Add C++17 structured bindings support for fixed-size Matrix and Array

Summary

• Specialize std::tuple_size and std::tuple_element for fixed-size Matrix and Array types
• Provide Eigen::get<Idx>() free functions found via ADL
• Guarded by EIGEN_MAX_CPP_VER >= 17 && EIGEN_COMP_CXXVER >= 17

Usage

Eigen::Vector3d v(1, 2, 3);
auto [x, y, z] = v;       // by-value decomposition
auto& [rx, ry, rz] = v;   // by-reference (mutation possible)

Eigen::Array3i a(4, 5, 6);
auto [a0, a1, a2] = a;    // works for Array too

Eigen::Matrix2d m;
m << 1, 2, 3, 4;
auto [m00, m10, m01, m11] = m;  // column-major order

Test plan

• Vector2, Vector3, Vector4 by-value decomposition (double, float, int)
• Array2, Array3 by-value decomposition
• Mutable reference binding (mutation through binding modifies original)
• Const reference binding
• Matrix2d decomposition (column-major order)
• 1x1 matrix edge case
• RowVector decomposition
• Static checks for tuple_size and tuple_element
• All existing tests pass

Closes #2247 (closed)