refactor(mir): Phase 137-6-S1 - choose_pattern_kind SSOT入口を新設

## 目的
Pattern 選択ロジックを SSOT 化し、将来の Canonicalizer 委譲に備える

## 変更内容

### 新規関数: `choose_pattern_kind()`
- 場所: `src/mir/builder/control_flow/joinir/routing.rs`
- 責務: Pattern 選択の SSOT 入口
- 実装: 既存の LoopFeatures ベース選択ロジックを集約

### LoopPatternContext の更新
- `new()` で `choose_pattern_kind()` を使用
- 既存の分散した選択ロジックを SSOT に統一

## 効果
- ✅ Pattern 選択ロジックの SSOT 化（1箇所に集約）
- ✅ 将来の Canonicalizer 委譲に備えた構造確立
- ✅ 既定挙動完全不変（既存テスト全て PASS）

## テスト結果
- ✅ `cargo build --release`: 成功
- ✅ スモークテスト（simple_*）: 5/5 PASS
- ✅ 退行なし

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/grammar/generated.rs

@@ -36,15 +36,37 @@ pub static OPERATORS_DIV_RULES: &[(&str, &str, &str, &str)] = &[
 ];
 pub fn lookup_keyword(word: &str) -> Option<&'static str> {
     for (k, t) in KEYWORDS {
-        if *k == word {
-            return Some(*t);
-        }
+        if *k == word { return Some(*t); }
     }
     None
 }
 
 pub static SYNTAX_ALLOWED_STATEMENTS: &[&str] = &[
-    "box", "global", "function", "static", "if", "loop", "break", "return", "print", "nowait",
-    "include", "local", "outbox", "try", "throw", "using", "from",
+    "box",
+    "global",
+    "function",
+    "static",
+    "if",
+    "loop",
+    "break",
+    "return",
+    "print",
+    "nowait",
+    "include",
+    "local",
+    "outbox",
+    "try",
+    "throw",
+    "using",
+    "from",
 ];
-pub static SYNTAX_ALLOWED_BINOPS: &[&str] = &["add", "sub", "mul", "div", "and", "or", "eq", "ne"];
+pub static SYNTAX_ALLOWED_BINOPS: &[&str] = &[
+    "add",
+    "sub",
+    "mul",
+    "div",
+    "and",
+    "or",
+    "eq",
+    "ne",
+];