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.

lib/ww/syntax/expr.ww

@@ -205,6 +205,17 @@ fn parseprimary(p: *parser) *node = {
 				mc.lhs = parsetype(p);
 			} else { if (p.curkind != tkind.TK_FATARROW) {
 				mc.lhs = parsetype(p);
+				// `case T1 | T2 | ... =>`: extra types chain
+				// through mc.list. Binding arms stay single-type
+				// (a binding over a multi-pattern would need the
+				// union type) — mirrors cmd/wcc/parse.c:643.
+				let alttail: *node = nil;
+				for (accepttok(p, tkind.TK_PIPE)) {
+					let more: *node = parsetype(p);
+					if (mc.list == nil) { mc.list = more; }
+					else { alttail.next = more; };
+					alttail = more;
+				};
 			};};
 			expecttok(p, tkind.TK_FATARROW, "expected '=>' in match arm");
 			mc.body = parsestmt(p);