refactor(naming): 箱理論 - Entry選択ロジック統一＆NamingBox統合

Phase 25.4 メンテナンス: Task A延長線上の箱化リファクタリング

## 📦 箱化内容

### 1. Entry選択ロジック統一箱
- 新規作成: `src/runner/modes/common_util/entry_selection.rs`
- 3箇所の重複ロジックを `select_entry_function()` に集約
  - `selfhost/json.rs:48-59` (11行削減)
  - `pyvm.rs:32-43` (11行削減)
  - `pyvm.rs:102-113` (11行削減)
  - `llvm.rs:292` (直接アクセス→統一関数呼び出し)

### 2. NamingBox SSOT 統合
- arity-aware優先フォールバック実装
  1. `Main.main/0` (arity付き、NYASH_BUILD_STATIC_MAIN_ENTRY=1時)
  2. `Main.main` (legacy、arity無し、現行デフォルト)
  3. `main` (top-level、NYASH_ENTRY_ALLOW_TOPLEVEL_MAIN=1時)
  4. `"Main.main"` (デフォルトフォールバック)

## 技術的成果
- **重複削減**: 33行の重複ロジック→1箇所に集約
- **NamingBox統一**: 全Entry選択が `src/mir/naming.rs` 経由
- **将来対応**: arity付き名前に自動対応（互換性維持）

## テスト結果
✅ cargo test mir_static_box_naming: 2 passed
✅ cargo test stage1_cli_entry_ssa_smoke: 2 passed

## 参考
- Phase 25.4-A完了commit: fa9cea51
- 箱理論原則: 重複ロジックを箱化→境界を明確化→一元管理

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

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

src/runner/modes/common_util/pyvm.rs

@@ -29,18 +29,8 @@ pub fn run_pyvm_harness(module: &crate::mir::MirModule, tag: &str) -> Result<i32
         tag,
         mir_json_path.display()
     );
-    // Determine entry function (prefer Main.main; top-level main only if allowed)
-    let allow_top = crate::config::env::entry_allow_toplevel_main();
-    let entry = if module.functions.contains_key("Main.main") {
-        "Main.main"
-    } else if allow_top && module.functions.contains_key("main") {
-        "main"
-    } else if module.functions.contains_key("main") {
-        eprintln!("[entry] Warning: using top-level 'main' without explicit allow; set NYASH_ENTRY_ALLOW_TOPLEVEL_MAIN=1 to silence.");
-        "main"
-    } else {
-        "Main.main"
-    };
+    // NamingBox SSOT: Select entry (arity-aware, Main.main → main fallback)
+    let entry = super::entry_selection::select_entry_function(&module);
     // Optional: Mini‑VM stdin loader — when enabled, read entire stdin and pass as argv[0]
     let mut cmd = std::process::Command::new(py3);
     crate::runner::child_env::apply_core_wrapper_env(&mut cmd);
@@ -58,7 +48,7 @@ pub fn run_pyvm_harness(module: &crate::mir::MirModule, tag: &str) -> Result<i32
             "--in",
             &mir_json_path.display().to_string(),
             "--entry",
-            entry,
+            &entry,
             "--args-env",
             "NYASH_SCRIPT_ARGS_JSON",
         ])
@@ -99,18 +89,8 @@ pub fn run_pyvm_harness_lib(module: &nyash_rust::mir::MirModule, tag: &str) -> R
         tag,
         mir_json_path.display()
     );
-    // Determine entry function (prefer Main.main; top-level main only if allowed)
-    let allow_top = crate::config::env::entry_allow_toplevel_main();
-    let entry = if module.functions.contains_key("Main.main") {
-        "Main.main"
-    } else if allow_top && module.functions.contains_key("main") {
-        "main"
-    } else if module.functions.contains_key("main") {
-        eprintln!("[entry] Warning: using top-level 'main' without explicit allow; set NYASH_ENTRY_ALLOW_TOPLEVEL_MAIN=1 to silence.");
-        "main"
-    } else {
-        "Main.main"
-    };
+    // NamingBox SSOT: Select entry (arity-aware, Main.main → main fallback)
+    let entry = super::entry_selection::select_entry_function(&module);
     let mut cmd = std::process::Command::new(py3);
     crate::runner::child_env::apply_core_wrapper_env(&mut cmd);
     if std::env::var("NYASH_MINIVM_READ_STDIN").ok().as_deref() == Some("1") {
@@ -126,7 +106,7 @@ pub fn run_pyvm_harness_lib(module: &nyash_rust::mir::MirModule, tag: &str) -> R
             "--in",
             &mir_json_path.display().to_string(),
             "--entry",
-            entry,
+            &entry,
             "--args-env",
             "NYASH_SCRIPT_ARGS_JSON",
         ])