phase29ad(p0): split pattern6 ok vs contract fixtures

apps/tests/phase29ab_pattern6_reverse_firstfail_min.hako

@@ -1,5 +1,5 @@
-// Phase 29ac P1: Pattern6 reverse scan OK minimal
-// Expect: last_index_bad_step("abc", "b") -> 1
+// Phase 29ad P0: Pattern6 reverse scan contract violation
+// Expect: JoinIR freeze (reverse step must be i = i - 1)
 
 static box Main {
     last_index_bad_step(s, ch) {
@@ -8,7 +8,7 @@ static box Main {
             if s.substring(i, i + 1) == ch {
                 return i
             }
-            i = i - 1
+            i = i - 2
         }
         return -1
     }