Implement export_report_xlsx(report) -> bytes: renders a SiteFinderReport (#987)
to .xlsx via openpyxl, one sheet per §13 section (Сводка/Рынок сейчас/Будущий
рынок/Продукт ТЗ/Сценарии/Скоринг). Deterministic, no LLM/DB/network; bytes via
BytesIO (no disk I/O). Accepts SiteFinderReport or its as_dict(). Graceful on
partial/empty/junk ("нет данных", never crashes). ADVISORY marker on Сводка.
11 tests (openpyxl reload, sheet titles, known cells, graceful). Additive — no
endpoint wiring (#959) or other exporters (#989).
