w6c: keep nested unwrap destinations distinct

selfhost/cmd/wcc/cgenexpr.ww

@@ -8780,7 +8780,11 @@ fn cgassign(c: *cgen, n: *syntax.node) void = {
 						os.write(2, m16.ptr, m16.len: u64);
 						os.exit(1);
 					};
-					let dstscr: i32 = localadd(c, "@unwrapdst", 8, nil);
+					// Fresh per lowering: cgexpr(rhs) may recursively
+					// emit another #16 assignment while this address is
+					// live. Sharing one per-fn slot would let the inner
+					// assignment overwrite the outer receiver.
+					let dstscr: i32 = localalloc(c, "@unwrapdst", 8, nil);
 					emitline("\tMOVQ\tBX, ");
 					emitoff(dstscr: i64);
 					emitline("(BP)\n");