4.14 sdds::Reader page indexing

• name:
  Reader::gotoPage, Reader::buildPageIndex, Reader::indexedPageCount
• description:
  Seek among pages on a seekable input. The reader lazily records page offsets during normal reads; gotoPage resumes from the nearest cached offset. buildPageIndex scans the input and eagerly caches every page offset.
• synopsis:
  void gotoPage(std::int64_t pageNumber);  
  void buildPageIndex();  
  std::optional<std::int64_t> indexedPageCount() const noexcept;

• arguments:
  • pageNumber — One-based page to return from the next read.
• return value:
  indexedPageCount returns the total count after complete indexing, or an empty optional while the count is not known. The other methods return nothing.
• exceptions:
  sdds::StateError for an unseekable source or invalid page number, sdds::LimitError when the index-entry limit is exceeded, or an I/O or format exception encountered while scanning.
• see also:
  Section 4.13.