w6c: return of a module-global struct ident copies the global's bytes

cstage zeroed the return scratch; mirror the landed wwstage emission
(copy from the g(SB) base). Flips the F8-era residual row to
cs==ww==correct. Task #42.

cmd/w6c/cgen.c

@@ -13675,12 +13675,21 @@ cgstmt(Cg *c, Node *n, Local **locals, int *frame)
 								    scr + idx * esz));
 							idx++;
 						}
-				} else if (n->lhs->kind == N_IDENT) {
+				} else if (n->lhs->kind == N_IDENT
+				    && localfind(*locals, n->lhs->str) != 0) {
 					/* N_IDENT: word-copy rhs slot into
 					 * scratch. Whole 8B words via MOVQ;
 					 * trailing partial word via MOVL/MOVB
 					 * so we read no further than the
-					 * source slot's declared size. */
+					 * source slot's declared size.
+					 *
+					 * #42 (#263): only a LOCAL ident has a
+					 * BP slot. A module-global struct ident
+					 * (localfind==0) falls to the addr-copy
+					 * else below — aggarg_srcaddr lands
+					 * g(SB) in SI. Pre-fix this read frame
+					 * garbage off 0(BP). ww half landed
+					 * F8-c6. */
 					int rhsoff = localfind(*locals,
 					    n->lhs->str);
 					int k = 0;