ToT: conj() on nested-tile contractions; real-plain x complex-ToT products via one real gemm

[kshitij-05]

Builds on #573 (branch kshitij/feature/strided-tot-kernels-complex); to be retargeted to master once #573 is merged.

Problem

1. conj() on a ToT contraction does not compile. conj(A * B) carries a ComplexConjugate<void> contraction factor; the ToT per-cell multiply-add ops in ContEngine static_cast that factor to the element type, Tensor::gemm forwards it as the BLAS alpha, and the ContractReduce<…, ComplexConjugate<…>> specializations name their result type through a value-returning gemm that nested tiles do not have.
2. A complex ToT contracted with a real plain tensor (A("i,j;a") * t("j,k"), Tensor<Tensor<complex<double>>> × Tensor<double>) skips the ToT × plain-tensor GEMM fast path, which requires identical element types, and runs the per-cell AXPY loop.

Changes

• detail::elem_factor<Numeric>(factor): the per-element multiplier for a contraction factor — the factor for numeric factors, 1 for ComplexConjugate<…> (conjugation and scale are applied to the finished result by ContractReduce's finalization, as for non-nested tiles). Used by the ToT per-cell ops in ContEngine and by Tensor::gemm's alpha.
• ContractReduce<…, ComplexConjugate<void>> / <…, ComplexConjugate<Scalar>>: result_type is Result, as in the primary template, and the nested-tile accumulate (which was an abort(); // not yet implemented stub) and the partial-result reduce are shared with the primary template through ContractReduceBase::accumulate_nested / reduce_results.
• ToT × real-plain products: when the plain element type is the real part of the inner element type, the complex slabs are viewed as real matrices with the inner extent doubled (re/im interleaved) and the existing strided GEMM runs in real arithmetic with alpha = beta = 1; results are identical to the complex product.
• Tests (tot_conj suite in tests/tot_expressions.cpp, real and complex rows): conj(a), permuted conj(a), conj(a) * b, a * conj(b), conj(a * b) (outer contraction with inner outer product), conj(a) * b with an inner contraction, and the mixed-type ToT × real-plain product, all against explicit references.

Testing

arena_strided_dgemm, einsum*, tot_expressions (incl. tot_conj), tot_dist_array_part{1,2}: 821 cases, no errors.

Not addressed

• Mixed real-ToT × complex-ToT products (the strided kernels require one element type for all three operands).