feat(mir): Phase 25.1g完了 - ControlForm導線追加（Rust側）

**実装内容**:
- ✅ Task G-1: If PHI wrapper (`merge_modified_with_control`) 追加
- ✅ Task G-2: Exit PHI wrapper (`build_exit_phis_for_control`) 追加
- ✅ 既存実装へのTODO/NOTEコメント追加（将来の統合導線確立）

**変更ファイル**:
- src/mir/phi_core/if_phi.rs: merge_modified_with_control() 追加
- src/mir/phi_core/loopform_builder.rs: build_exit_phis_for_control() 追加
- src/mir/loop_builder.rs: TODO/NOTEコメント追加（2箇所）

**テスト結果**:
- ✅ mir_stage1_using_resolver_min_fragment_verifies
- ✅ mir_stage1_using_resolver_full_collect_entries_verifies
- ✅ mir_stageb_loop_break_continue (2 tests)
- ✅ mir_loopform_exit_phi (4 tests)
- ⚠️ test_stageb_min.sh Test2は既知の問題（Phase 25.1g前から）

**設計方針**:
- Thin wrapper pattern: ControlFormを受け取り既存実装に委譲
- 挙動変更なし: SSA/PHI生成ロジックは完全に既存のまま
- 観測のみ: NYASH_IF_TRACE/NYASH_LOOPFORM_DEBUGでControlForm使用をログ
- 段階移行準備: 将来の統合時に切り替え導線が明確

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

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

src/mir/loop_builder.rs

@@ -368,6 +368,10 @@ impl<'a> LoopBuilder<'a> {
         // Exit block
         self.set_current_block(exit_id)?;
 
+        // NOTE(25.1g+): ControlForm 統合後は build_exit_phis_for_control(...) 経由で
+        // LoopShape から exit_id/branch_source_block を決める導線に寄せる予定。
+        // 現在は既存実装のまま（挙動変更なし）。
+
         // Build exit PHIs for break statements
         let exit_snaps = self.exit_snapshots.clone();
         loopform.build_exit_phis(self, exit_id, branch_source_block, &exit_snaps)?;
@@ -1176,6 +1180,10 @@ impl<'a> LoopBuilder<'a> {
         // Reset to pre-if snapshot, then delegate to shared helper
         self.parent_builder.variable_map = pre_if_var_map.clone();
         let mut ops = Ops(self);
+
+        // TODO(25.1g+): ControlForm 統合版に切り替えるときは、
+        // IfShape から ControlForm を渡して merge_modified_with_control() を呼ぶ。
+        // 現在は既存実装のまま（挙動変更なし）。
         crate::mir::phi_core::if_phi::merge_modified_at_merge_with(
             &mut ops,
             merge_bb,

src/mir/phi_core/if_phi.rs

@@ -241,3 +241,55 @@ pub fn merge_with_reset_at_merge_with<O: PhiMergeOps>(
         skip_var,
     )
 }
+
+/// Phase 25.1g: ControlForm-based wrapper for If PHI generation.
+/// This provides a thin adapter layer that accepts ControlForm and delegates
+/// to the existing merge_modified_at_merge_with implementation.
+///
+/// **Important**: This does NOT change any PHI generation logic - it merely
+/// provides a ControlForm-based entry point while preserving existing behavior.
+pub fn merge_modified_with_control<O: PhiMergeOps>(
+    ops: &mut O,
+    form: &crate::mir::control_form::ControlForm,
+    pre_if_snapshot: &HashMap<String, ValueId>,
+    then_map_end: &HashMap<String, ValueId>,
+    else_map_end_opt: &Option<HashMap<String, ValueId>>,
+    skip_var: Option<&str>,
+    // Existing implementation requires pred info, so we accept it as parameters
+    then_pred_opt: Option<crate::mir::BasicBlockId>,
+    else_pred_opt: Option<crate::mir::BasicBlockId>,
+) -> Result<(), String> {
+    use crate::mir::control_form::ControlKind;
+
+    // Only process If structures; silently succeed for other control kinds
+    let shape = match &form.kind {
+        ControlKind::If(shape) => shape,
+        _ => return Ok(()),
+    };
+
+    // Extract merge_bb from IfShape
+    let merge_bb = shape.merge_block;
+
+    // Log ControlForm usage when trace is enabled
+    let trace = std::env::var("NYASH_IF_TRACE").ok().as_deref() == Some("1");
+    if trace {
+        eprintln!(
+            "[if-phi/control-form] Using ControlForm wrapper: merge={:?} then={:?} else={:?}",
+            merge_bb, shape.then_block, shape.else_block
+        );
+    }
+
+    // Delegate to existing implementation - no behavioral changes
+    merge_modified_at_merge_with(
+        ops,
+        merge_bb,
+        shape.then_block,
+        shape.else_block.unwrap_or(shape.then_block),
+        then_pred_opt,
+        else_pred_opt,
+        pre_if_snapshot,
+        then_map_end,
+        else_map_end_opt,
+        skip_var,
+    )
+}

src/mir/phi_core/loopform_builder.rs

@@ -479,6 +479,44 @@ fn sanitize_phi_inputs(inputs: &mut Vec<(BasicBlockId, ValueId)>) {
     *inputs = vec;
 }
 
+/// Phase 25.1g: ControlForm-based wrapper for LoopForm Exit PHI generation.
+/// This provides a thin adapter layer that accepts ControlForm and delegates
+/// to the existing LoopFormBuilder::build_exit_phis implementation.
+///
+/// **Important**: This does NOT change any PHI generation logic - it merely
+/// provides a ControlForm-based entry point while preserving existing behavior.
+pub fn build_exit_phis_for_control<O: LoopFormOps>(
+    loopform: &LoopFormBuilder,
+    ops: &mut O,
+    form: &crate::mir::control_form::ControlForm,
+    exit_snapshots: &[(crate::mir::BasicBlockId, std::collections::HashMap<String, crate::mir::ValueId>)],
+    // Existing implementation requires branch_source_block, so we accept it as a parameter
+    branch_source_block: crate::mir::BasicBlockId,
+) -> Result<(), String> {
+    use crate::mir::control_form::ControlKind;
+
+    // Only process Loop structures; silently succeed for other control kinds
+    let shape = match &form.kind {
+        ControlKind::Loop(shape) => shape,
+        _ => return Ok(()),
+    };
+
+    // Extract exit_id from LoopShape
+    let exit_id = shape.exit;
+
+    // Log ControlForm usage when trace is enabled
+    let trace = std::env::var("NYASH_LOOPFORM_DEBUG").ok().is_some();
+    if trace {
+        eprintln!(
+            "[loopform/exit-phi/control-form] Using ControlForm wrapper: exit={:?} branch_source={:?}",
+            exit_id, branch_source_block
+        );
+    }
+
+    // Delegate to existing implementation - no behavioral changes
+    loopform.build_exit_phis(ops, exit_id, branch_source_block, exit_snapshots)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;