Unify the three `evaluate()` overloads via a descriptor template

[MoonFlowww]

Motivation

Nott's public API promise is a clean, safe surface. Right now core.hpp exposes three structurally identical evaluate() overloads that differ only in descriptor type. Adding a fourth evaluation mode (e.g., regression, ranking) requires copy-pasting the same boilerplate again. The /// TODO: do it cleaner comment at line 2398 acknowledges this debt.

Current State

src/core.hpp:2385–2423 has three overloads:

auto evaluate(..., Evaluation::ClassificationDescriptor, ...)      -> ClassificationReport;
auto evaluate(..., Evaluation::MultiClassificationDescriptor, ...)  -> ClassificationReport; // TODO: do it cleaner
auto evaluate(..., Evaluation::SegmentationDescriptor, ...)         -> ClassificationReport;

All three bodies are identical:

return Evaluation::Evaluate(*this, inputs, targets, descriptor, metrics, options);

~30 lines of pure duplication with no behavioural difference at this layer.

Proposed Change

Collapse to a single constrained template:

template<Evaluation::EvaluationDescriptor DescriptorT>
auto evaluate(torch::Tensor inputs,
              torch::Tensor targets,
              DescriptorT   descriptor,
              std::vector<Metric::Classification::Descriptor> metrics,
              Evaluation::Options options = {}) -> Evaluation::ClassificationReport
{
    return Evaluation::Evaluate(*this,
        std::move(inputs), std::move(targets),
        descriptor, std::move(metrics), options);
}

Where Evaluation::EvaluationDescriptor is a concept:

template<typename T>
concept EvaluationDescriptor =
    std::same_as<T, Evaluation::ClassificationDescriptor>      ||
    std::same_as<T, Evaluation::MultiClassificationDescriptor> ||
    std::same_as<T, Evaluation::SegmentationDescriptor>;

Acceptance Criteria

• Single evaluate() template replaces the three overloads
• EvaluationDescriptor concept defined and constrains the template
• Passing an unsupported descriptor type produces a clear compile-time error
• /// TODO: do it cleaner comment removed
• All existing call sites compile without changes