* feat(query): add VectorViewClause zero-copy path and unify validate
- Add VectorViewClause (string_view-based) as zero-copy counterpart to
VectorClause; variant now holds VectorClause | VectorViewClause | FtsClause
- Add QueryTarget::get_vector_view() unified accessor via std::visit,
returns optional<VectorViewClause> regardless of which variant is held
- Split validate_and_sanitize into QueryTarget::validate (read-only) +
sanitize_sparse_vector (mutate); validate handles both VectorClause and
VectorViewClause via get_vector_view()
- Collection::Query passes original request directly to sqlengine when no
sparse sanitization is needed; only copies when sort is required
- Change build_query_info/BuildSQLInfoFromSearchQuery to take const
SearchQuery& so VectorMatrixNode string_views point to caller's data
egolearner