test(joinir): Phase 88 - Fail-Fast validation for non-const i update

- Add test: rejects_non_const_then_i_update
- Validates that continue pattern requires `i = i + const` form
- Ensures Fail-Fast behavior for unsupported step patterns

Impact:
- 60 normalized_dev tests PASS (+1)
- Fail-Fast 仕様固定（非 const の i 更新を拒否）

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

tests/normalized_joinir_min/shapes.rs

@@ -313,6 +313,65 @@ fn test_phase88_jsonparser_unescape_string_step2_min_canonical_matches_structure
     assert_eq!(canonical, JoinValue::Int(5));
 }
 
+/// Phase 88: Continue 側の `i` 更新は `i = i + const` のみに限定して Fail-Fast する。
+#[test]
+fn test_phase88_jsonparser_unescape_string_step2_min_rejects_non_const_then_i_update() {
+    use std::any::Any;
+
+    fn panic_message(payload: Box<dyn Any + Send>) -> String {
+        if let Some(s) = payload.downcast_ref::<&str>() {
+            return (*s).to_string();
+        }
+        if let Some(s) = payload.downcast_ref::<String>() {
+            return s.clone();
+        }
+        "<non-string panic payload>".to_string()
+    }
+
+    // then 側の `i = i + n`（const ではない）を入れて、Fail-Fast を確認する。
+    let program_json = serde_json::json!({
+      "version": 0,
+      "kind": "Program",
+      "defs": [{
+        "type": "FunctionDef",
+        "name": "jsonparser_unescape_string_step2_min",
+        "params": ["n"],
+        "body": { "type": "Block", "body": [
+          { "type": "Local", "name": "i", "expr": { "type": "Int", "value": 0 } },
+          { "type": "Local", "name": "acc", "expr": { "type": "Int", "value": 0 } },
+          { "type": "Loop",
+            "cond": { "type": "Compare", "op": "<", "lhs": { "type": "Var", "name": "i" }, "rhs": { "type": "Var", "name": "n" } },
+            "body": [
+              { "type": "If",
+                "cond": { "type": "Compare", "op": "<", "lhs": { "type": "Var", "name": "i" }, "rhs": { "type": "Var", "name": "n" } },
+                "then": [
+                  { "type": "Local", "name": "i", "expr": { "type": "Binary", "op": "+", "lhs": { "type": "Var", "name": "i" }, "rhs": { "type": "Var", "name": "n" } } },
+                  { "type": "Continue" }
+                ],
+                "else": []
+              },
+              { "type": "Local", "name": "acc", "expr": { "type": "Binary", "op": "+", "lhs": { "type": "Var", "name": "acc" }, "rhs": { "type": "Int", "value": 0 } } },
+              { "type": "Local", "name": "i", "expr": { "type": "Binary", "op": "+", "lhs": { "type": "Var", "name": "i" }, "rhs": { "type": "Int", "value": 1 } } }
+            ]
+          },
+          { "type": "Return", "expr": { "type": "Var", "name": "acc" } }
+        ]}
+      }]
+    });
+
+    let res = std::panic::catch_unwind(|| {
+        let mut lowerer = nyash_rust::mir::join_ir::frontend::AstToJoinIrLowerer::new();
+        lowerer.lower_program_json(&program_json);
+    });
+    assert!(res.is_err(), "expected fail-fast panic");
+    let msg = panic_message(res.err().unwrap());
+    assert!(
+        msg.contains("then i update of form (i + const)"),
+        "unexpected panic message: {}",
+        msg
+    );
+}
+
 /// Phase 48-C: JsonParser _parse_object continue skip_ws canonical route should match Structured
 #[test]
 fn test_normalized_pattern4_jsonparser_parse_object_continue_skip_ws_canonical_matches_structured()