docs(feature-count-sweep): add experimental sweep page + surface at entry points New page: - Feature-Count-Sweep.md — full coverage of FeatureCountSweep and sweep_feature_count with a prominent Experimental warning, Quick Start snippet, forecast-vs-cv mode table, estimator_mode trade-off, fail-fast behaviour on missing prediction-matrix features, EvaluationModel prediction-reuse wiring (with build_label shortcut), 1-SE parsimony rule, complete example, and full constructor / function reference. Cross-links + surfaces: - Home.md — new nav row (marked Experimental). - Training-Workflow.md — Post-Hoc top_n_features Sweep sub-section with a minimal runnable snippet and link to the deep-dive page. - API-Reference.md — FeatureCountSweep (Experimental) section covering constructor, .fit(...), sweep_feature_count(...), and persistence. - Architecture.md — feature_count_sweep.py added to the features/ module listing. - Output-Structure.md — sweep outputs nested under training/features/{cv-slug}/sweep/{mode}/{classifier-name}/ and a matching row in the What Lives Where cheat sheet. Also captures accumulated smaller edits since the last wiki push (evaluation, explanation, output-structure clarifications).