syntax+w6c: multi-type match patterns in wwstage (align up to cstage)

`case T1 | T2 =>` (binding-less; cstage parse.c:635-650 chains extra
types through cs->list, bindings stay single-type by design). Three
wwstage layers, no AST change (node.list already exists):
- parser (expr.ww): pipe loop after the non-let arm's first
  parsetype;
- checker (check.ww): resolvewalk N_MCASE now walks n.list so each
  alt gets its type_ stamp (exhaustiveness/casecovers were already
  alt-aware, built ahead of the parser);
- cgen (cgenexpr.ww): the single-pattern want-computation moves
  VERBATIM into matcharmwant; a cs.list arm emits CMPQ/JE per alt
  funneling into one match_body label, mirroring cgen.c:11118-11141
  incl. label mint order and the tag<0 clamp. Nullable arms keep
  ignoring alts (both stages).

Graduates the held e2e row (pin 1732/1158/3464); byte-identical on
the repro and the single-pattern control.

selfhost/cmd/wcc/check.ww

@@ -671,6 +671,14 @@ fn resolvewalk(c: *checker, n: *syntax.node) void = {
 	// Mirrors cmd/wcc/check.c's newscope/saved-restore around cstmt.
 	if (k == syntax.nkind.N_MCASE) {
 		if (n.lhs  != nil) { resolvewalk(c, n.lhs);  };
+		// `case T1 | T2` alts ride n.list; each needs its type_
+		// stamp or cgmatch's tag lookup collapses to 0. Mirrors
+		// cmd/wcc/check.c:2140 (per-alt resolve).
+		let alt: *syntax.node = n.list;
+		for (alt != nil) {
+			resolvewalk(c, alt);
+			alt = alt.next;
+		};
 		let outer: *syntax.scope = c.cur;
 		c.cur = syntax.newscope(outer);
 		let nm: str = n.str;