Two halves, Node only. The helper, against fake responses: a real WebP
is accepted by type or by its bytes; a host's 200 fallback page is
refused as a raster and as a source; HTML labelled octet-stream is
refused; a 404 and a network error are misses for the raster; a network
error on the source is thrown as itself. The callers: neither
usePosterPrints nor PosterWall may fetch posters/ directly again, since
each once carried its own copy and a guard in one did not reach the other.
Poison-tested: restoring the old accept-anything fetch fails 3 checks,
and PosterWall's direct fetches failed the caller check before its switch.
gates:pre: PASS 67, FAIL 0.
