With posterSources on, the tree shipped only the 28 zone poster sources,
so exporting any of the 176 city or 174 country posters said "poster not
found (404)": 350 of 378 posters could not be printed.
Each poster now ships once: its .html source where one exists, else its
.webp. Generating sources for the 350 was not an option: city posters
need OSM street tiles to regenerate (publish-html.mjs), and a source is
the heavy form (about 1 MB each against about 0.1 MB for a raster).
Measured with check:tree-weight: 102.7 MB before, 137.9 MB after, of
160 MB. The 350 rasters are the tracked data/posters/*.webp: all 350 are
existing git blobs, so the repository gains no new pack bytes.
New gate check:posters-exportable (in gates:post). Every poster id, from
data/posters and the wall's previews, must ship a real WebP or a real
poster source (its class="stage" element), so a placeholder or a host's
fallback page cannot pass. It skips when posters are off by config, which
check:poster-degrades covers. Poison-tested: a missing poster and a fake
image each fail.
