refactor(joinir): Refactor-A+B - Null literal + ContinueReturn 一般化

## Refactor-A: Null literal 対応
- expr.rs に "Null" case 追加
- ConstValue::Null → JoinValue::Unit マッピング

## Refactor-B: ContinueReturn 一般化
- 複数 return-if 許可（同値のみ）
- json_values_equal() ヘルパー追加
- Fail-Fast: 異なる値は即エラー

## Fixtures & Tests
- null_literal_min.program.json (return null)
- continue_return_multi_min.program.json (複数 return null)
- +1 test (vm_bridge 検証)

Impact:
- normalized_dev: 63→64 passed (+1)
- lib: 993 passed (回帰なし)
- 箱化スコア: 5/5 (全項目満点)
- 実ループ(_parse_array/_parse_object) 準備完了

🤖 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

@@ -599,3 +599,20 @@ fn test_parse_string_composite_min_expected_output() {
         "Expected acc=5 for n=10 (i=0,1,2,5,6 increments acc, i=3 escape continue, i=7 close quote return)"
     );
 }
+
+/// Refactor-A+B: ContinueReturn multi minimal - tests both Null literal and multiple return-if
+#[test]
+fn test_continue_return_multi_min_returns_null_at_first_match() {
+    use nyash_rust::mir::join_ir::normalized::dev_fixtures::NormalizedDevFixture;
+
+    let _ctx = normalized_dev_test_ctx();
+    let structured = NormalizedDevFixture::ContinueReturnMultiMin.load_and_lower();
+    let entry = structured.entry.expect("entry required");
+
+    let input = [JoinValue::Int(10)];
+    let result = run_joinir_vm_bridge(&structured, entry, &input, true);
+    // Tests:
+    // - Refactor-A: Null literal support (returns ConstValue::Null → JoinValue::Unit)
+    // - Refactor-B: Multiple return-if with same value (i==3, i==7 both return null)
+    assert_eq!(result, JoinValue::Unit, "Expected Unit (null) from first return-if at i=3");
+}