Function binaryheap-from-vecpub

Convert a (Vec elem) to a (BinaryHeap elem). Spec mandates linear-time heapify; the WP-F3 partial body is an identity cast at the runtime layer (the cell layout is already a Vec). Heap-invariant restoration arrives when the real sift-up/sift-down intrinsics land — callers that rely on the max-heap invariant MUST NOT depend on this behaviour yet. Free fn rather than method because method dispatch on (Vec elem) would collide across VecDeque, LinkedList, BinaryHeap et al.