epubveri library for calibre 0.2.0

• The scan validates several books at once, and you choose how many.
  DNSB scanned about 18 000 books in 1 h 29 min on an idle system
  (MobileRead 375207 #2, #8) and the whole of that was one book at a time.

  How much faster depends entirely on your machine, so this note does not
  give you a number to compare yours against. Not every core is worth the
  same — a laptop with a mix of fast and efficient cores gains noticeably less
  than its core count suggests — and the scan is limited by the processor
  rather than by the disk, so a slow drive is not what holds it back.

  The other reason is that we cannot see your machine. A Books validated at
  the same time control is in Preferences → Plugins → Customize, and its
  maximum is your physical core count less two: two are kept for the rest of
  the system, calibre included. Physical rather than logical cores, because on
  a hyperthreaded 8-core machine the logical count is 16 and reserving two of
  those would still leave fourteen processes fighting over eight cores.

  It defaults to Automatic, which is that maximum, or the number of books
  if the selection is smaller — a fourth worker on three books has nothing to
  do.

  An estimate that also read free memory and the size of the largest books was
  written and then removed: measuring it showed a scan costs far less memory
  than it was insuring against, and the library it would have protected — one
  of large illustrated books, bulk-scanned on a small laptop — is one nobody
  has reported. If yours behaves that way, say so and it can be built against
  a real library rather than a guess about one.

• It is listed as the GUI plugin it is. Its category was forced to "Edit
  book tool" so that it would sit beside the editor plugin in Preferences →
  Plugins — one product, one heading — with the known cost that the heading
  named where its sibling lives rather than where it does.

  MobileRead settled that the other way: a moderator retitled the thread
  [GUI Plugin] epubveri library, and it is filed under Extend calibre
  generally. That is calibre's own community answering the same question, so
  a Preferences entry reading "Edit book tool" would now disagree with the
  index a user found the plugin in. It takes its own category, and the two
  plugins name each other in their descriptions instead — which the category
  never could do anyway.

  Nothing functional turns on it: all three uses in calibre are cosmetic.

• What deliberately did not change. Cancelling still stops the scan
  handing out new books and keeps everything already done — the books in
  flight are allowed to finish and are counted rather than discarded. One
  pathological archive still costs only its own timeout. And the report is
  identical whatever the worker count: the book list is filled by each book's
  own position rather than in the order results happen to arrive, and the rule
  rows were already safe because their sort key ends in the rule's own code.
  Both are asserted by tests that were checked by breaking the guarantee and
  watching them fail.