refactor(joinir): L-2.2 Step-1 - Unify Stage-B lowering via LoopToJoinLowerer

Stage-B minimal JoinIR lowering統一化:

1. Add Case-A helpers to loop_to_join.rs:
   - lower_case_a_for_stageb_body (StageBBodyExtractorBox.build_body_src/2)
   - lower_case_a_for_stageb_funcscanner (StageBFuncScannerBox.scan_all_boxes/1)

2. Update stageb_body.rs and stageb_funcscanner.rs:
   - Replace manual LoopForm construction with construct_simple_while_loopform
   - Route through LoopToJoinLowerer like other Case-A functions
   - Remove unused imports (LoopForm, MirQuery)

Now all 6 JoinIR lowering modules use the same unified pattern:
- skip_ws, trim, append_defs, stage1_resolver, stageb_body, stageb_funcscanner
All route through LoopToJoinLowerer for consistency.

Test results: JSON snapshot 6/6 PASS

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

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

src/mir/join_ir/lowering/loop_to_join.rs

@@ -339,6 +339,26 @@ impl LoopToJoinLowerer {
     ) -> Option<JoinModule> {
         self.lower(func, loop_form, Some("Stage1UsingResolverBox.resolve_for_source/5"))
     }
+
+    /// Case-A 汎用 lowerer の「StageBBodyExtractorBox.build_body_src/2 用」薄いラッパー。
+    /// 実際のロジックは `lower` に集約されている。
+    pub fn lower_case_a_for_stageb_body(
+        &self,
+        func: &MirFunction,
+        loop_form: &LoopForm,
+    ) -> Option<JoinModule> {
+        self.lower(func, loop_form, Some("StageBBodyExtractorBox.build_body_src/2"))
+    }
+
+    /// Case-A 汎用 lowerer の「StageBFuncScannerBox.scan_all_boxes/1 用」薄いラッパー。
+    /// 実際のロジックは `lower` に集約されている。
+    pub fn lower_case_a_for_stageb_funcscanner(
+        &self,
+        func: &MirFunction,
+        loop_form: &LoopForm,
+    ) -> Option<JoinModule> {
+        self.lower(func, loop_form, Some("StageBFuncScannerBox.scan_all_boxes/1"))
+    }
 }
 
 #[cfg(test)]

src/mir/join_ir/lowering/stageb_body.rs

@@ -11,8 +11,7 @@ use crate::mir::join_ir::lowering::common::{
 };
 use crate::mir::join_ir::lowering::value_id_ranges::stageb_body_extract as vid;
 use crate::mir::join_ir::JoinModule;
-use crate::mir::loop_form::LoopForm;
+use crate::mir::query::MirQueryBox;
-use crate::mir::query::{MirQuery, MirQueryBox};
 
 /// Public dispatcher (MIR-based vs handwritten)
 pub fn lower_stageb_body_to_joinir(module: &crate::mir::MirModule) -> Option<JoinModule> {
@@ -173,22 +172,30 @@ fn lower_from_mir(module: &crate::mir::MirModule) -> Option<JoinModule> {
         return lower_handwritten(module);
     }
 
+    // Phase 32: LoopToJoinLowerer 統一箱経由に移行
+    // construct_simple_while_loopform 共通ヘルパーを使用
     if crate::mir::join_ir::env_flag_is_1("NYASH_JOINIR_LOWER_GENERIC") {
-        let header = query.succs(entry).get(0).copied().unwrap_or(entry);
+        use crate::mir::join_ir::lowering::common::construct_simple_while_loopform;
-        let succs_header = query.succs(header);
+        use crate::mir::join_ir::lowering::loop_to_join::LoopToJoinLowerer;
-        let body = succs_header.get(0).copied().unwrap_or(header);
+
-        let exit = succs_header.get(1).copied().unwrap_or(header);
+        // stageb_body: entry_is_preheader=true, has_break=true
-        let loop_form = LoopForm {
+        let Some(loop_form) = construct_simple_while_loopform(entry, &query, true, true) else {
-            preheader: entry,
+            eprintln!("[joinir/stageb_body/generic-hook] failed to construct LoopForm from CFG");
-            header,
+            return build_stageb_body_joinir(module);
-            body,
-            latch: body,
-            exit,
-            continue_targets: vec![body],
-            break_targets: vec![exit],
         };
+
         if crate::mir::join_ir::lowering::common::case_a::is_simple_case_a_loop(&loop_form) {
-            eprintln!("[joinir/stageb_body/generic-hook] simple Case A loop detected (generic_case_a hook placeholder)");
+            eprintln!(
+                "[joinir/stageb_body/generic-hook] simple Case A loop detected (LoopToJoinLowerer)"
+            );
+            let lowerer = LoopToJoinLowerer::new();
+            if let Some(jm) = lowerer.lower_case_a_for_stageb_body(target_func, &loop_form) {
+                eprintln!("[joinir/stageb_body/generic-hook] LoopToJoinLowerer produced JoinIR, returning early");
+                return Some(jm);
+            }
+            eprintln!(
+                "[joinir/stageb_body/generic-hook] LoopToJoinLowerer returned None, falling back to handwritten"
+            );
         }
     }
 

src/mir/join_ir/lowering/stageb_funcscanner.rs

@@ -11,8 +11,7 @@ use crate::mir::join_ir::lowering::common::{
 };
 use crate::mir::join_ir::lowering::value_id_ranges::stageb_funcscanner as vid;
 use crate::mir::join_ir::JoinModule;
-use crate::mir::loop_form::LoopForm;
+use crate::mir::query::MirQueryBox;
-use crate::mir::query::{MirQuery, MirQueryBox};
 
 /// Public dispatcher (MIR-based vs handwritten)
 pub fn lower_stageb_funcscanner_to_joinir(module: &crate::mir::MirModule) -> Option<JoinModule> {
@@ -163,22 +162,30 @@ fn lower_from_mir(module: &crate::mir::MirModule) -> Option<JoinModule> {
         return lower_handwritten(module);
     }
 
+    // Phase 32: LoopToJoinLowerer 統一箱経由に移行
+    // construct_simple_while_loopform 共通ヘルパーを使用
     if crate::mir::join_ir::env_flag_is_1("NYASH_JOINIR_LOWER_GENERIC") {
-        let header = query.succs(entry).get(0).copied().unwrap_or(entry);
+        use crate::mir::join_ir::lowering::common::construct_simple_while_loopform;
-        let succs_header = query.succs(header);
+        use crate::mir::join_ir::lowering::loop_to_join::LoopToJoinLowerer;
-        let body = succs_header.get(0).copied().unwrap_or(header);
+
-        let exit = succs_header.get(1).copied().unwrap_or(header);
+        // stageb_funcscanner: entry_is_preheader=true, has_break=true
-        let loop_form = LoopForm {
+        let Some(loop_form) = construct_simple_while_loopform(entry, &query, true, true) else {
-            preheader: entry,
+            eprintln!("[joinir/stageb_funcscanner/generic-hook] failed to construct LoopForm from CFG");
-            header,
+            return build_stageb_funcscanner_joinir(module);
-            body,
-            latch: body,
-            exit,
-            continue_targets: vec![body],
-            break_targets: vec![exit],
         };
+
         if crate::mir::join_ir::lowering::common::case_a::is_simple_case_a_loop(&loop_form) {
-            eprintln!("[joinir/stageb_funcscanner/generic-hook] simple Case A loop detected (generic_case_a hook placeholder)");
+            eprintln!(
+                "[joinir/stageb_funcscanner/generic-hook] simple Case A loop detected (LoopToJoinLowerer)"
+            );
+            let lowerer = LoopToJoinLowerer::new();
+            if let Some(jm) = lowerer.lower_case_a_for_stageb_funcscanner(target_func, &loop_form) {
+                eprintln!("[joinir/stageb_funcscanner/generic-hook] LoopToJoinLowerer produced JoinIR, returning early");
+                return Some(jm);
+            }
+            eprintln!(
+                "[joinir/stageb_funcscanner/generic-hook] LoopToJoinLowerer returned None, falling back to handwritten"
+            );
         }
     }