epubveri library for calibre 0.1.0

First version. Validates a calibre library with epubveri and reports the
defects that recur across it.

• A rule-first report. One row per defect, ordered by how many books carry
  it, rather than a list of books with error counts. The three measurements
  behind that choice are in the README; the short version is that a
  book-and-count report ranks the least interesting book first.
• Whole library or selection, as a background job with progress in the Jobs
  panel. Cancelling keeps what the scan already found.
• Show affected books marks the books a row names and filters the library
  view to them, so the way out of the report is into calibre's own view — and
  from there into Edit Book, where the editor plugin says what is wrong in
  place.
• Export or copy the report as CSV, carrying the grouping fields
  (violation_kind, the construct named) as well as the counts.
• Errors and fatals are always ranked; warnings by default. Usage notes and
  advisory findings are fetched but not ranked unless asked for, and the window
  says how many findings the filter is hiding.
• Its own copy of the validator, in plugins/epubveri-library-data/,
  independent of the editor plugin's. Sharing one was tried first and dropped:
  a scan holds the binary open for ten minutes, Windows does not allow a
  running executable to be overwritten, and the other plugin's hourly update
  check would land in the middle of that. The install record sits beside the
  binary, so deleting the folder leaves nothing behind that describes a file
  that is gone.

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Install: calibre → Preferences → Plugins → Load plugin from file, then restart calibre. Add it to a toolbar under Preferences → Toolbars & menus if calibre does not offer to.

It appears beside the epubveri Edit Book plugin in the plugin list, and its toolbar button says Validate library. The two are one product on two surfaces: this one answers which books have a problem, that one answers what the problem is.

Verify what you downloaded against SHA256SUMS.txt beside the zip.

Only run on calibre 9.14, macOS. minimum_calibre_version is 6.0 because that is the oldest release it can load on, not a claim about testing.