feat(joinir): Phase 33-9.1 Loop/If lowering responsibility separation

Implement clear separation between Loop and If lowering responsibilities:

**Guard Implementation:**
- Add is_loop_lowered_function() to identify 6 Loop-dedicated functions
- Exclude Loop functions from If lowering path in try_lower_if_to_joinir()
- Enforce "1 function → 1 lowering" principle

**Documentation:**
- Add responsibility comments to loop_to_join.rs
- Add responsibility comments to if_select.rs and if_merge.rs
- Update if_joinir_design.md with Phase 33-9.1 section

**Testing:**
- Add unit test test_is_loop_lowered_function() (PASS)
- Verify no regression in existing JoinIR tests

**Loop-dedicated functions (6):**
- Main.skip/1
- FuncScannerBox.trim/1
- FuncScannerBox.append_defs/2
- Stage1UsingResolverBox.resolve_for_source/5
- StageBBodyExtractorBox.build_body_src/2
- StageBFuncScannerBox.scan_all_boxes/1

This prevents future conflicts when both Loop and If lowering expand.

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

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

src/mir/join_ir/lowering/mod.rs

@@ -47,6 +47,34 @@ pub use stageb_funcscanner::lower_stageb_funcscanner_to_joinir;
 use crate::mir::join_ir::JoinInst;
 use crate::mir::{BasicBlockId, MirFunction};
 
+/// Phase 33-9.1: Loop lowering対象関数の判定
+///
+/// これらの関数は Phase 32/33 で LoopToJoinLowerer によって処理されます。
+/// If lowering (Select/IfMerge) の対象から除外することで、Loop/If の責務を明確に分離します。
+///
+/// ## 対象関数（6本）
+/// - Main.skip/1: 空白スキップループ
+/// - FuncScannerBox.trim/1: 前後空白削除ループ
+/// - FuncScannerBox.append_defs/2: 配列結合ループ
+/// - Stage1UsingResolverBox.resolve_for_source/5: using解析ループ
+/// - StageBBodyExtractorBox.build_body_src/2: Stage-B本体抽出ループ
+/// - StageBFuncScannerBox.scan_all_boxes/1: Stage-B Box走査ループ
+///
+/// ## 将来の拡張
+/// NYASH_JOINIR_LOWER_GENERIC=1 で汎用 Case-A ループにも拡張可能
+pub(crate) fn is_loop_lowered_function(name: &str) -> bool {
+    const LOOP_LOWERED_FUNCTIONS: &[&str] = &[
+        "Main.skip/1",
+        "FuncScannerBox.trim/1",
+        "FuncScannerBox.append_defs/2",
+        "Stage1UsingResolverBox.resolve_for_source/5",
+        "StageBBodyExtractorBox.build_body_src/2",
+        "StageBFuncScannerBox.scan_all_boxes/1",
+    ];
+
+    LOOP_LOWERED_FUNCTIONS.contains(&name)
+}
+
 /// Phase 33-7: Try to lower if/else to JoinIR Select/IfMerge instruction
 ///
 /// Scope:
@@ -73,6 +101,13 @@ pub fn try_lower_if_to_joinir(
         return None;
     }
 
+    // Phase 33-9.1: Loop専任関数の除外（Loop/If責務分離）
+    // Loop lowering対象関数はIf loweringの対象外にすることで、
+    // 1関数につき1 loweringの原則を保証します
+    if is_loop_lowered_function(&func.signature.name) {
+        return None;
+    }
+
     // Phase 33-8: デバッグログレベル取得（0-3）
     let debug_level = crate::config::env::joinir_debug_level();
     let _debug = debug || debug_level >= 1;
@@ -148,3 +183,40 @@ pub fn try_lower_if_to_joinir(
 
     result
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Phase 33-9.1: is_loop_lowered_function() の動作確認
+    #[test]
+    fn test_is_loop_lowered_function() {
+        // Loop 専任関数（6本）は true を返す
+        assert!(is_loop_lowered_function("Main.skip/1"));
+        assert!(is_loop_lowered_function("FuncScannerBox.trim/1"));
+        assert!(is_loop_lowered_function("FuncScannerBox.append_defs/2"));
+        assert!(is_loop_lowered_function(
+            "Stage1UsingResolverBox.resolve_for_source/5"
+        ));
+        assert!(is_loop_lowered_function(
+            "StageBBodyExtractorBox.build_body_src/2"
+        ));
+        assert!(is_loop_lowered_function(
+            "StageBFuncScannerBox.scan_all_boxes/1"
+        ));
+
+        // If lowering 対象関数は false を返す
+        assert!(!is_loop_lowered_function("IfSelectTest.simple_return/0"));
+        assert!(!is_loop_lowered_function("IfMergeTest.multiple_true/0"));
+        assert!(!is_loop_lowered_function(
+            "JsonShapeToMap._read_value_from_pair/1"
+        ));
+        assert!(!is_loop_lowered_function(
+            "Stage1JsonScannerBox.value_start_after_key_pos/2"
+        ));
+
+        // 一般的な関数も false を返す
+        assert!(!is_loop_lowered_function("SomeBox.some_method/3"));
+        assert!(!is_loop_lowered_function("Main.main/0"));
+    }
+}