selfhost: port append + spread — N_SPREAD parser + rt_ensure builtin

lib/ww/parse/expr.ww

@@ -217,6 +217,14 @@ fn parsearglist(p: *parser, closekind: tkind, headout: **node) void = {
 	let tail: *node = nil;
 	for (true) {
 		let e: *node = parseexpr(p);
+		// Hare-style spread: `expr...` in an arg slot becomes a
+		// marker the callee/builtin can iterate over. Mirrors
+		// cmd/wcc/parse.c. The only consumer today is `append`.
+		if (accepttok(p, tkind.TK_ELLIPSIS)) {
+			let sp: *node = newnode(p.a, nkind.N_SPREAD, e.file, e.line, e.col);
+			sp.lhs = e;
+			e = sp;
+		};
 		if (head == nil) { head = e; tail = e; }
 		else { tail.next = e; tail = e; };
 		if (!accepttok(p, tkind.TK_COMMA)) { break; };