A fillet or chamfer on a cylindrical boss's bottom arc now builds with a closed display mesh: the seam-extension containment ray counts a shared curved-boundary endpoint once. Merging the split faces of a three-edge arc corner rebuilds the face on the original input face's full surface instead of a piece's restricted patch, and runs under the kernel's panic guard. Fixes #1262 and #1263. Co-authored-by: Karl-VonRichter <[email protected]>