Tip bitmaps for every brush file this workspace reads.
preview_abr, preview_brush and preview_brushset each take the whole
file as bytes and return one [PreviewEntry] per brush in file order,
available or not. A brush whose tip cannot be rendered is reported with a
reason rather than dropped, so a caller can lay out a complete grid.
Every function here is pure over &[u8]: no filesystem, no threads, so the
crate builds for wasm32-unknown-unknown.