refactor(pattern2): move promotion decision into pattern2/api SSOT

Phase 263 P0.2: pattern2/api/ フォルダ化 - 入口SSOTの物理固定

目的: PromoteDecision/try_promote の参照点を1箇所に閉じ込めて、迷子防止

Changes:
1. 新規フォルダ構造:
   - pattern2/api/mod.rs - 入口SSOT（try_promote と PromoteDecision を再export）
   - pattern2/api/promote_decision.rs - PromoteDecision/PromoteStepResult 型定義
   - pattern2/api/promote_runner.rs - try_promote(...) 実装（SSOT entry point）
   - pattern2/mod.rs - api モジュールを公開

2. 移動/抽出:
   - PromoteDecision enum を promote_step_box.rs → pattern2/api/promote_decision.rs へ
   - promote_and_prepare_carriers を try_promote として pattern2/api/promote_runner.rs へ抽出

3. promote_step_box.rs を薄いラッパに縮退（35行, -177行）:
   - pattern2::api::try_promote を呼び出すだけの互換用ラッパ
   - 将来的に削除予定（deprecated）

4. orchestrator を新API に書き換え:
   - use super::pattern2::api::{try_promote, PromoteDecision};
   - try_promote(...) を直接呼び出し

5. 参照確認:
   - rg で確認: すべての参照が pattern2::api 経由に収束 ✅

検証:
- cargo test --lib: 1368/1368 PASS ✅
- quick smoke: 45/46 PASS ✅（悪化なし）
- 参照点が pattern2/api に一本化され、迷子防止を物理的に保証

src/mir/builder/control_flow/joinir/patterns/mod.rs

@@ -60,6 +60,7 @@ pub(in crate::mir::builder) mod escape_pattern_recognizer; // Phase 91 P5b
 pub(in crate::mir::builder) mod common_init;
 pub(in crate::mir::builder) mod loop_true_counter_extractor; // Phase 104: loop(true) counter extraction for Pattern2
 pub(in crate::mir::builder) mod read_digits_break_condition_box; // Phase 104: break cond normalization for read_digits(loop(true))
+pub(in crate::mir::builder) mod pattern2; // Phase 263 P0.2: Pattern2 module (api/ SSOT entry point)
 pub(in crate::mir::builder) mod pattern2_break_condition_policy_router; // Phase 105: policy router box for Pattern2 break condition
 pub(in crate::mir::builder) mod pattern2_policy_router; // Phase 108: unified Pattern2 policy router (balanced/read_digits/default)
 pub(in crate::mir::builder) mod pattern2_inputs_facts_box; // Phase 105: Pattern2 input facts (analysis only)