A sparse metadata.opf/metadata.json (title only, no description) left
books thin even when the file itself carried rich data: the embedded
extractors only ran when no sidecar existed at all. Now mergeMetadata
fills blanks from the book's own OPF - title, author, description,
publisher, language, ISBN, ASIN, series/number, publish date, tags,
contributors - while sidecar values always win and unparseable files
skip silently. Also covers .kepub, which the merge previously ignored
while the extractor already supported it.
Adds TestMergeMetadataEPUBGapFill asserting both directions: sidecar
title/author survive, embedded description/publisher/language fill in.
John O'Keefe
708598687e