Add hcat/vcat DenseBase concatenation expressions

Summary

Add hcat() and vcat() free functions for concatenating dense expressions horizontally and vertically.

Design:

• New ConcatOp<Direction, LhsType, RhsType> expression class following the Replicate.h pattern
• internal::traits specialization with compile-time dimension propagation
• Custom evaluator with coeff(row, col) dispatching to lhs/rhs based on boundary index
• Uses ref_selector in the expression + nested_eval in the evaluator for proper rvalue lifetime handling
• Binary-only API (no variadic) for CUDA device code compatibility
• EIGEN_DEVICE_FUNC on all methods

Files:

• Eigen/src/Core/ConcatOp.h — expression class, traits, evaluator, hcat()/vcat() free functions
• Eigen/src/Core/util/ForwardDeclarations.h — forward declaration
• Eigen/Core — include directive
• test/concat.cpp — 13 test suites (1062 lines)
• test/CMakeLists.txt — test registration

Test plan

Local Docker (GCC 15, CMake 3.31, Debian trixie, C++17):

#	Test suite	What it covers
1	test_concat_dynamic	Dynamic-size vcat/hcat with float, double, complex float; different-sized operands
2	test_concat_with_expressions	Scalar multiply, sum/diff, transpose, block expression inputs
3	test_concat_fixed	Fixed-size matrices; compile-time RowsAtCompileTime/ColsAtCompileTime verification
4	test_concat_mixed_fixed_dynamic	Fixed+dynamic → Dynamic rows; size_prefer_fixed cols propagation
5	test_concat_rvalue_temporaries	Product temporaries, additive temporaries, auto storage, nested chains
6	test_concat_chained	vcat(vcat(a,b),c), hcat(hcat(a,b),c), mixed vcat→hcat, 4-way chain
7	test_concat_vectors	Column vectors, row vectors, fixed-size Vector3+Vector4, RowVector3+RowVector4
8	test_concat_array	Dynamic/fixed-size Arrays, coefficient-wise expression inputs, XprKind propagation
9	test_concat_coeff_access	Element-by-element verification of every coefficient
10	test_concat_in_expressions	Matrix multiply on concat result, .sum(), .squaredNorm(), assignment to block
11	test_concat_single_row_col	Single-row vcat, single-column hcat, 1×1 matrices
12	test_concat_row_major	RowMajor+RowMajor, RowMajor+ColMajor, fixed-size RowMajor
13	test_concat_self	Self-concat vcat(a,a), hcat(a,a), vector self-concat, self-expression concat

Results: 13/13 test suites pass, 0 compiler warnings (GCC 15 -Wall -Wextra), each suite tested with both float and double.

Also verified no regressions in basicstuff, product_small, block tests.

Addresses #2691