lib/strings: add dup, Hare-shape

lib/strings/strings.ww

@@ -106,3 +106,23 @@ export fn concat(a: str, b: str) str = {
 	r.len = total;
 	return r;
 };
+
+// dup — duplicate a string into a fresh allocation. Caller owns the
+// returned str's storage; release via `os.free(r.ptr, r.len)`. Mirrors
+// Hare's strings::dup shape — Hare returns `(str | nomem)`, ww doesn't
+// have nomem (os.alloc aborts on OOM), so we return plain `str`.
+//
+// Empty input yields a `{nil, 0}` str — Hare returns the static empty
+// string; same observable result.
+export fn dup(s: str) str = {
+	let r: str;
+	r.ptr = nil;
+	r.len = 0;
+	if (s.len == 0) { return r; };
+	let buf: *u8 = os.alloc(s.len: u64): *u8;
+	let i: i32 = 0;
+	for (i < s.len) { buf[i] = s[i]; i += 1; };
+	r.ptr = buf;
+	r.len = s.len;
+	return r;
+};