w6c: tagged GLOBAL reassign emits the store

cstage silently dropped the store on reassigning a module-global
tagged union; mirror the landed wwstage emission (tag+payload to
g(SB) via the widener). Closes the cs half of the F8 #263 pair; the
repro row flips to cs==ww==correct. Task #41.

cmd/w6c/cgen.c

@@ -6909,7 +6909,27 @@ cgexpr(Cg *c, Node *n, Local *locals)
 				if (!rhs_sret_call) {
 					int off = localfind(locals,
 					    n->lhs->str);
-					if (off == 0) break;
+					if (off == 0) {
+						/* #41 (#263): a module-global
+						 * tagged ident has no BP slot —
+						 * LEAQ g(SB),BX then store
+						 * tag+payload off BX (mirror the
+						 * global-struct-field tagged arm
+						 * above + the local arm below).
+						 * Pre-fix this break dropped the
+						 * store entirely; ww half landed
+						 * in F8-c5. */
+						if (let_islet(n->lhs->str)) {
+							ins2(c, A_LEAQ,
+							    masym(c, n->lhs->str),
+							    areg(D_BX));
+							cg_widen_tagged_store(c,
+							    &locals, lu, n->rhs,
+							    D_BX, 0,
+							    (int)lu->size);
+						}
+						break;
+					}
 					cg_widen_tagged_store(c, &locals, lu,
 					    n->rhs, D_BP, off, (int)lu->size);
 					break;