Epsilon

A numerical tolerance, threaded via using through Hyperplane.from/Hypersphere/HelixDelaunay's internal geometric comparisons (near-zero singular values, near-zero circumsphere margins). Deliberately NOT a package- level given (the earlier shape here): Hyperplane/Hypersphere/HelixDelaunay are never constructed from outside this file (every external caller goes through AlphaShapes.apply below), so the one default value this codebase actually uses only needs to live on apply's own using parameter -- an ordinary default value, not an ambient given every file that happens to wildcard-import alpha.{given, *} would otherwise pick up silently.