Interface MeshDistance

Minimum-distance query, exposed here rather than left package-private because every consumer that wants it lives outside this package. The viewer's measure tool needs the exact surface-to-surface distance, and before this it could not reach any of the machinery: triTriDistance sits under math/, which has no export subpath, so a deep import is hard-blocked by ERR_PACKAGE_PATH_NOT_EXPORTED.

buildMeshBvh / queryMeshCross come with it: a caller measuring one element against several should build each tree once rather than pay for it per query.