Same gap-fill rule as the EPUB path, against the PDF's embedded Info
dictionary via a readPDFInfoDict helper reusing extractPDFMetadata's
field conventions (creator falls back to author, subject maps to
description, producer to publisher, keywords to tags, plus page count).
Sidecar values always win; unopenable PDFs skip silently.
TestMergeMetadataPDFGapFill uses an Info-bearing hand-built PDF fixture
(shared with the sidecar-cover test) and asserts the sidecar title is
kept while author/description/publisher/tags/page count fill in.
John O'Keefe
e944f11415