Function btreemap-from-hashmappub

.from-hashmap converts a (HashMap k v) into a (BTreeMap k v). Contents are identical; only the ordering discipline changes. The WP-F3 partial body is an identity cast at the runtime layer (the cell layout is already a HashMap). Ordered-iteration semantics arrive when the real B-tree intrinsics land — callers that rely on ordered traversal MUST NOT depend on this behaviour yet. Free fn rather than method because method dispatch on (HashMap k v) would collide with other HashMap-wrapping newtypes in the future.