wcc/cgen: S1 loud-stop subset-cast tagged widen in let+return (wwstage align to cstage #35)

A widening sub-union cast (return/let `v: inner` into a union whose member
is the nested `inner`) skipped the nested widen arm and emitted tag=0 (a
2nd-variant value returned the wrong payload). cstage loud-stops #35;
wwstage now matches: cgwidentaggedstorebp louds on the subset cast, and
cgreturn routes genuine-widening tagged returns (rhs TY_TAGGED, ru1!=fu1)
through the widener choke-point so the same loud fires. Same-type returns
stay on the proven passthrough. Faithful inner-tag->outer-index remap
deferred. test/wcc/835 S1 row.

selfhost/cmd/wcc/cgenstmt.ww

@@ -1037,6 +1037,29 @@ fn cgreturn(c: *cgen, n: *node) void = {
 					if (taggedmemread(c, rhs)) {
 						needswiden = true;
 					};
+					// S1/#35: a GENUINE-WIDENING tagged source
+					// (stamped type_ TY_TAGGED and != fnret;
+					// exact-type rides forwardtagged/plain above)
+					// routes through the widener — tag remap for
+					// ident/call (#218), #35 widen-subset loud for
+					// cast/dot. Mirrors cstage cgreturn istagged→
+					// cg_widen_tagged_store (cmd/w6c/cgen.c:13008-
+					// 13011 → :2721). The ru1 != fu1 exclusion keeps
+					// EXACT-type tagged casts on cstage's passthrough
+					// (forwardtagged's own ru==fu equality) so byte-id
+					// holds.
+					let ru1: *tinfo = rhs.type_: *tinfo;
+					ru1 = tichase(ru1);
+					let fu1: *tinfo = nil;
+					if (c.fnret != nil) {
+						fu1 = c.fnret.type_: *tinfo;
+						fu1 = tichase(fu1);
+					};
+					if (ru1 != nil && fu1 != nil) {
+						if (ru1.kind == tykind.TY_TAGGED && ru1 != fu1) {
+							needswiden = true;
+						};
+					};
 				};
 			};
 			if (needswiden) {