feat(joinir): Phase 48-2 from_loop_form method for Trio absorption

- Add LoopScopeShape::from_loop_form() that creates Trio internally
- Remove LoopExitLivenessBox import from loop_to_join.rs
- Switch loop_to_join.rs to use from_loop_form() instead of from_existing_boxes()

This is the first step in absorbing Classifier Trio into LoopScopeShape.
External Trio dependency reduced from 3 boxes to 2 (intake_loop_form only).

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

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

src/mir/join_ir/lowering/loop_scope_shape.rs

@@ -253,6 +253,41 @@ impl LoopScopeShape {
         Self::from_existing_boxes_legacy(loop_form, intake, var_classes, exit_live_box, query)
     }
 
+    /// Phase 48-2: Trio引数なしでLoopScopeShapeを構築
+    ///
+    /// # Purpose
+    ///
+    /// 外部からTrioを渡す必要をなくし、LoopScopeShape内部に完全吸収。
+    /// `loop_to_join.rs` などで空のTrioを作成して渡すパターンを解消する。
+    ///
+    /// # Arguments
+    ///
+    /// - `loop_form`: LoopForm containing block structure
+    /// - `intake`: LoopFormIntake containing classified variable info
+    /// - `query`: MirQuery for liveness computation
+    /// - `func_name`: Optional function name for Case-A minimal routing
+    ///
+    /// # Returns
+    ///
+    /// Some(LoopScopeShape) if successful, None if critical data is missing.
+    ///
+    /// # History
+    ///
+    /// - Phase 48-2: 新規追加（Trio依存の内部化）
+    pub(crate) fn from_loop_form(
+        loop_form: &LoopForm,
+        intake: &LoopFormIntake,
+        query: &impl MirQuery,
+        func_name: Option<&str>,
+    ) -> Option<Self> {
+        // Phase 48-2: Trio を内部で作成（空箱）
+        let var_classes = LoopVarClassBox::new();
+        let exit_live_box = LoopExitLivenessBox::new();
+
+        // 既存の from_existing_boxes を呼び出し
+        Self::from_existing_boxes(loop_form, intake, &var_classes, &exit_live_box, query, func_name)
+    }
+
     /// Check if a variable needs header PHI
     #[allow(dead_code)] // Phase 30: will be used in F-3 generic lowering
     pub fn needs_header_phi(&self, var_name: &str) -> bool {

src/mir/join_ir/lowering/loop_to_join.rs

@@ -31,8 +31,7 @@ use crate::mir::join_ir::lowering::loop_form_intake::intake_loop_form;
 use crate::mir::join_ir::lowering::loop_scope_shape::LoopScopeShape;
 use crate::mir::join_ir::JoinModule;
 use crate::mir::loop_form::LoopForm;
-use crate::mir::phi_core::loop_exit_liveness::LoopExitLivenessBox;
-use crate::mir::phi_core::loop_var_classifier::LoopVarClassBox;
+// Phase 48-2: Trio imports removed - now internal to LoopScopeShape::from_loop_form()
 use crate::mir::query::MirQueryBox;
 use crate::mir::MirFunction;
 
@@ -98,22 +97,13 @@ impl LoopToJoinLowerer {
         // Step 1: MirQuery を構築
         let query = MirQueryBox::new(func);
 
-        // Step 2: 分類箱を構築（Phase 31 では空箱、将来 MIR 解析で埋める）
-        let var_classes = LoopVarClassBox::new();
-        let exit_live = LoopExitLivenessBox::new();
+        // Step 2: LoopFormIntake を構築
+        // Phase 48-2: intake_loop_form は空の var_classes を使用（将来は from_loop_form に統合）
+        let intake = intake_loop_form(loop_form, &Default::default(), &query, func)?;
 
-        // Step 3: LoopFormIntake を構築
-        let intake = intake_loop_form(loop_form, &var_classes, &query, func)?;
-
-        // Step 4: LoopScopeShape を構築
-        let scope = LoopScopeShape::from_existing_boxes(
-            loop_form,
-            &intake,
-            &var_classes,
-            &exit_live,
-            &query,
-            func_name,
-        )?;
+        // Step 3: LoopScopeShape を構築
+        // Phase 48-2: from_loop_form() で Trio を内部化（LoopExitLivenessBox 依存削除）
+        let scope = LoopScopeShape::from_loop_form(loop_form, &intake, &query, func_name)?;
 
         if self.debug {
             eprintln!(