Toggle the JRPlaceholder fallback based on the real poster's load state. Mirrors the canonical state machine from JRRowItem.bs onPosterLoadStatusChanged.

Flip the placeholder into failure state with a type-appropriate glyph, then clear the poster URI so the glyph isn't covered by a stale broken image. itemType="" puts JRPlaceholder in loading state (backdrop only, no glyph); resolveItemPlaceholderType returns "" only for invalid/typeless items, so valid items always end up here with a typed glyph or the Folder fallback.