refactor(mir): loop_builder.rs モジュール化 - 6ファイルに分割

## リファクタリング内容

### ファイル構造変更
- `src/mir/loop_builder.rs` (1515行) 削除
- `src/mir/loop_builder/` ディレクトリ新設（6ファイル、1529行）

### 新規モジュール構成

1. **mod.rs** (6,293行 → 実際は約150行)
   - モジュール定義とre-export
   - LoopBuilder構造体定義

2. **loop_form.rs** (25,988行 → 実際は約650行)
   - メインループlowering pipeline
   - デバッグ/実験フラグ集約

3. **if_lowering.rs** (15,600行 → 実際は約390行)
   - In-loop if lowering with JoinIR/PHI bridge
   - **Phase 61-2コード完全保持**:
     - JoinIR dry-run検証モード
     - PhiSpec計算とA/B比較

4. **phi_ops.rs** (12,844行 → 実際は約320行)
   - PHI emit helpers
   - LoopFormOps/PhiBuilderOps impls

5. **control.rs** (4,261行 → 実際は約107行)
   - break/continue capture
   - predecessor bookkeeping

6. **statements.rs** (1,673行 → 実際は約42行)
   - loop-body statement lowering entry point

7. **README.md** (752行 → 実際は約19行)
   - モジュール責務とサブモジュール説明

### 設計原則

- **責務分離**: CFG構築/PHI生成/制御フロー/文処理を分離
- **Phase 61-2保持**: if_lowering.rsにJoinIR dry-run完全移行
- **phi_core委譲**: PHI構築ロジックは`phi_core`に委譲

## テスト結果

- Phase 61-2テスト: ✅ 2/2 PASS（dry-runフラグ、PhiSpec）
- loopformテスト: ✅ 14/14 PASS（退行なし）
- ビルド: ✅ 成功（エラー0件）

## 統計

- **純削減**: -1,521行（25ファイル変更）
- **loop_builder**: 1515行 → 1529行（+14行、6ファイル化）
- **可読性**: 巨大単一ファイル → 責務別モジュール

## ChatGPT設計・Claude確認

大規模リファクタリングをChatGPTが実施、Claudeが検証完了。

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

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

src/mir/builder/control_flow.rs

@@ -81,10 +81,16 @@ impl super::MirBuilder {
         // Note: Arity does NOT include implicit `me` receiver
         let is_target = match func_name.as_str() {
             "JsonTokenizer.print_tokens/0" => {
-                std::env::var("HAKO_JOINIR_PRINT_TOKENS_MAIN").ok().as_deref() == Some("1")
+                std::env::var("HAKO_JOINIR_PRINT_TOKENS_MAIN")
+                    .ok()
+                    .as_deref()
+                    == Some("1")
             }
             "ArrayExtBox.filter/2" => {
-                std::env::var("HAKO_JOINIR_ARRAY_FILTER_MAIN").ok().as_deref() == Some("1")
+                std::env::var("HAKO_JOINIR_ARRAY_FILTER_MAIN")
+                    .ok()
+                    .as_deref()
+                    == Some("1")
             }
             _ => false,
         };
@@ -133,10 +139,10 @@ impl super::MirBuilder {
         debug: bool,
     ) -> Result<Option<ValueId>, String> {
         use super::loop_frontend_binding::LoopFrontendBinding;
-        use crate::r#macro::ast_json::ast_to_json;
         use crate::mir::join_ir::frontend::{AstToJoinIrLowerer, JoinFuncMetaMap};
         use crate::mir::join_ir_vm_bridge::convert_join_module_to_mir_with_meta;
         use crate::mir::types::ConstValue;
+        use crate::r#macro::ast_json::ast_to_json;
 
         // Phase 50: Create appropriate binding based on function name
         let binding = match func_name {
@@ -156,16 +162,13 @@ impl super::MirBuilder {
         if debug {
             eprintln!(
                 "[cf_loop/joinir] Using binding: counter={}, acc={:?}, pattern={:?}",
-                binding.counter_var,
-                binding.accumulator_var,
-                binding.pattern
+                binding.counter_var, binding.accumulator_var, binding.pattern
             );
         }
 
         // Step 1: Convert condition and body to JSON
         let condition_json = ast_to_json(condition);
-        let mut body_json: Vec<serde_json::Value> =
-            body.iter().map(|s| ast_to_json(s)).collect();
+        let mut body_json: Vec<serde_json::Value> = body.iter().map(|s| ast_to_json(s)).collect();
 
         // Phase 50: Rename variables in body (e.g., "out" → "acc" for filter)
         binding.rename_body_variables(&mut body_json);
@@ -192,7 +195,10 @@ impl super::MirBuilder {
                 continue;
             }
             if debug {
-                eprintln!("[cf_loop/joinir] Adding '{}' to params (external_ref)", ext_ref);
+                eprintln!(
+                    "[cf_loop/joinir] Adding '{}' to params (external_ref)",
+                    ext_ref
+                );
             }
             params.push(serde_json::json!(ext_ref));
         }
@@ -370,7 +376,8 @@ impl super::MirBuilder {
         }
 
         // 3. Collect all ValueIds used in JoinIR function
-        let mut used_values: std::collections::BTreeSet<ValueId> = std::collections::BTreeSet::new();
+        let mut used_values: std::collections::BTreeSet<ValueId> =
+            std::collections::BTreeSet::new();
         for block in join_func.blocks.values() {
             Self::collect_values_in_block(block, &mut used_values);
         }
@@ -420,7 +427,9 @@ impl super::MirBuilder {
                                 );
                             }
                         }
-                        MirInstruction::Jump { target: exit_block_id }
+                        MirInstruction::Jump {
+                            target: exit_block_id,
+                        }
                     }
                     _ => Self::remap_instruction(term, &value_map, &block_map),
                 };
@@ -503,7 +512,9 @@ impl super::MirBuilder {
                 values.insert(*value);
                 values.insert(*ptr);
             }
-            MirInstruction::Call { dst, func, args, .. } => {
+            MirInstruction::Call {
+                dst, func, args, ..
+            } => {
                 if let Some(d) = dst {
                     values.insert(*d);
                 }
@@ -512,7 +523,9 @@ impl super::MirBuilder {
                     values.insert(*arg);
                 }
             }
-            MirInstruction::BoxCall { dst, box_val, args, .. } => {
+            MirInstruction::BoxCall {
+                dst, box_val, args, ..
+            } => {
                 if let Some(d) = dst {
                     values.insert(*d);
                 }
@@ -595,24 +608,39 @@ impl super::MirBuilder {
                 value: remap_value(*value),
                 ptr: remap_value(*ptr),
             },
-            MirInstruction::Call { dst, func, callee, args, effects } => MirInstruction::Call {
+            MirInstruction::Call {
+                dst,
+                func,
+                callee,
+                args,
+                effects,
+            } => MirInstruction::Call {
                 dst: dst.map(remap_value),
                 func: remap_value(*func),
                 callee: callee.clone(),
                 args: args.iter().map(|a| remap_value(*a)).collect(),
                 effects: *effects,
             },
-            MirInstruction::BoxCall { dst, box_val, method, method_id, args, effects } => {
-                MirInstruction::BoxCall {
-                    dst: dst.map(remap_value),
-                    box_val: remap_value(*box_val),
-                    method: method.clone(),
-                    method_id: *method_id,
-                    args: args.iter().map(|a| remap_value(*a)).collect(),
-                    effects: *effects,
-                }
-            }
-            MirInstruction::Branch { condition, then_bb, else_bb } => MirInstruction::Branch {
+            MirInstruction::BoxCall {
+                dst,
+                box_val,
+                method,
+                method_id,
+                args,
+                effects,
+            } => MirInstruction::BoxCall {
+                dst: dst.map(remap_value),
+                box_val: remap_value(*box_val),
+                method: method.clone(),
+                method_id: *method_id,
+                args: args.iter().map(|a| remap_value(*a)).collect(),
+                effects: *effects,
+            },
+            MirInstruction::Branch {
+                condition,
+                then_bb,
+                else_bb,
+            } => MirInstruction::Branch {
                 condition: remap_value(*condition),
                 then_bb: remap_block(*then_bb),
                 else_bb: remap_block(*else_bb),
@@ -634,7 +662,11 @@ impl super::MirBuilder {
                 dst: remap_value(*dst),
                 src: remap_value(*src),
             },
-            MirInstruction::NewBox { dst, box_type, args } => MirInstruction::NewBox {
+            MirInstruction::NewBox {
+                dst,
+                box_type,
+                args,
+            } => MirInstruction::NewBox {
                 dst: remap_value(*dst),
                 box_type: box_type.clone(),
                 args: args.iter().map(|a| remap_value(*a)).collect(),