Function btreeset-from-vecpub

Convert a (Vec elem) to a (BTreeSet elem). Spec mandates duplicate collapse and ascending-order iteration; the WP-F3 partial body is an identity cast at the runtime layer (the cell layout is already a Vec). Deduplication and sort arrive when the real balanced-tree intrinsics land — callers that rely on uniqueness or ordering MUST NOT depend on this behaviour yet. Free fn rather than method because method dispatch on (Vec elem) would collide across VecDeque, LinkedList, BinaryHeap, BTreeSet et al.