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)]