Render PDF headlines as document cards, not lightbox imagesHEADmaster

A starred PDF previously ran through the same crop-and-lightbox path
a photo does -- exactly the awkward treatment explicit headline
designation was meant to avoid. _headline_image.html.erb now branches
on @headline_asset.pdf?: a PDF renders as a linked card (a :medium
thumbnail, a file icon, its name), no gallery participation at all.
The existing image-headline and gallery-fallback logic is otherwise
unchanged, now scoped to an image-specific headline only.

Other attached PDFs -- headlined or not -- list below as plain links,
same reasoning as why non-headline photos still get a gallery
trigger: an attached document shouldn't go invisible just because
nothing's been starred yet.

headline_image's own render guard needed widening to cover a page
with PDFs attached but no images and no headline at all -- the one
case none of the existing conditions accounted for.

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