Document + pin F017: zhang_suen erases isolated 2x2 blocks An isolated 2x2 block is a genuine divergence point between the parallel algorithms and matters for small-blob masks (marker remnants, dotted-line dashes, segmentation specks). All four block pixels satisfy the zhang_suen deletion gate (B == 3, A == 1, both sub-iteration corner products zero) in the same sub-iteration, so the default method erases the block entirely, while guo_hall keeps exactly one pixel. Document the property in vignette("choosing-a-method") -- a caveat on the zhang_suen/guo_hall bullets plus a new "Small isolated blobs" subsection with a runnable example and the mitigation (use guo_hall or size-filter blobs before thinning). Add a tripwire-style pinned test asserting zhang_suen -> empty and guo_hall -> one pixel; if either count changes, the vignette guidance is wrong and must be updated with it. Also records the F014 hilditch fix in NEWS. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>#8