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.

test/wcc/data/r700_match_multi_pattern/case.ww

@@ -0,0 +1,27 @@
+//ww:run-exit 18
+// Migrated from 700_e2e row 72.
+package main;
+fn pick(n: i64) (i64 | i32 | u32) = {
+  if (n < 0) { return 1: i32; };
+  if (n == 0) { return 2: u32; };
+  return n;
+};
+fn main() i32 = {
+  let r1: (i64 | i32 | u32) = pick(0);
+  let r2: (i64 | i32 | u32) = pick(-1);
+  let r3: (i64 | i32 | u32) = pick(7);
+  let acc: i32 = 0;
+  match (r1) {
+    case let v: i64 => acc += 100;
+    case i32 | u32 => acc += 1;
+  };
+  match (r2) {
+    case let v: i64 => acc += 100;
+    case i32 | u32 => acc += 10;
+  };
+  match (r3) {
+    case let v: i64 => acc += v: i32;
+    case i32 | u32 => acc += 100;
+  };
+  return acc;
+};