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>

src/mir/join_ir/normalized/dev_fixtures.rs

@@ -21,6 +21,9 @@ pub enum NormalizedDevFixture {
     PatternContinueReturnMin,
     /// Parse String Composite minimal (Phase 90 P0)
     ParseStringCompositeMin,
+    /// Refactor-B: ContinueReturn multi minimal (multiple return-if with same value)
+    /// Note: This also tests Null literal support from Refactor-A
+    ContinueReturnMultiMin,
 }
 
 impl NormalizedDevFixture {
@@ -36,6 +39,7 @@ impl NormalizedDevFixture {
             }
             Self::PatternContinueReturnMin => "pattern_continue_return_minimal",
             Self::ParseStringCompositeMin => "parse_string_composite_minimal",
+            Self::ContinueReturnMultiMin => "continue_return_multi_minimal",
         }
     }
 
@@ -57,6 +61,9 @@ impl NormalizedDevFixture {
             Self::ParseStringCompositeMin => {
                 "../../../../docs/private/roadmap2/phases/normalized_dev/fixtures/parse_string_composite_min.program.json"
             }
+            Self::ContinueReturnMultiMin => {
+                "../../../../docs/private/roadmap2/phases/normalized_dev/fixtures/continue_return_multi_min.program.json"
+            }
         }
     }
 
@@ -68,7 +75,8 @@ impl NormalizedDevFixture {
             | Self::Pattern4JsonParserParseArrayContinueSkipWs
             | Self::Pattern4JsonParserParseObjectContinueSkipWs
             | Self::PatternContinueReturnMin
-            | Self::ParseStringCompositeMin => FunctionRoute::LoopFrontend,
+            | Self::ParseStringCompositeMin
+            | Self::ContinueReturnMultiMin => FunctionRoute::LoopFrontend,
         }
     }
 
@@ -93,6 +101,9 @@ impl NormalizedDevFixture {
             Self::ParseStringCompositeMin => include_str!(
                 "../../../../docs/private/roadmap2/phases/normalized_dev/fixtures/parse_string_composite_min.program.json"
             ),
+            Self::ContinueReturnMultiMin => include_str!(
+                "../../../../docs/private/roadmap2/phases/normalized_dev/fixtures/continue_return_multi_min.program.json"
+            ),
         }
     }
 
@@ -122,6 +133,7 @@ pub const ALL_DEV_FIXTURES: &[NormalizedDevFixture] = &[
     NormalizedDevFixture::Pattern4JsonParserParseObjectContinueSkipWs,
     NormalizedDevFixture::PatternContinueReturnMin,
     NormalizedDevFixture::ParseStringCompositeMin,
+    NormalizedDevFixture::ContinueReturnMultiMin,
 ];
 
 #[cfg(test)]
@@ -156,7 +168,7 @@ mod tests {
     }
 
     #[test]
-    fn test_pattern4_fixtures_route_to_loop_frontend() {
+    fn test_all_fixtures_route_to_loop_frontend() {
         for fixture in ALL_DEV_FIXTURES {
             assert_eq!(
                 fixture.route(),