feat(joinir): Phase 27.8-4/5 — skip_ws CFG sanity checks + fallback design

- MirQuery API を使用した軽量 CFG チェック実装
  - Entry block successors >= 1
  - Const(0) 存在確認
  - String.length() 存在確認
- 予期しない MIR 構造時の手書き版 fallback 設計
- A/B テスト完了: 手書き版・MIR-based 版両方 PASS ✅
- ドキュメント更新: joinir_coverage.md + IMPLEMENTATION_LOG.md

Phase 27.9 modular refactoring: commit 3d5979c7
Phase 27.8-4/5 verification: CFG checks + docs updates

src/mir/join_ir/lowering/skip_ws.rs

@@ -229,6 +229,9 @@ fn lower_skip_ws_handwritten(module: &crate::mir::MirModule) -> Option<JoinModul
 /// ## 実装状況:
 /// - Phase 27.8: 基本実装（MirQuery を使用した MIR 解析）
 fn lower_skip_ws_from_mir(module: &crate::mir::MirModule) -> Option<JoinModule> {
+    use crate::mir::query::{MirQuery, MirQueryBox};
+    use crate::mir::MirInstruction;
+
     // Step 1: "Main.skip/1" を探す
     let target_func = module.functions.get("Main.skip/1")?;
 
@@ -246,6 +249,34 @@ fn lower_skip_ws_from_mir(module: &crate::mir::MirModule) -> Option<JoinModule>
         return lower_skip_ws_handwritten(module);
     }
 
+    // Phase 27.8-4: Lightweight CFG sanity checks
+    // MirQueryBox を使ってパターンマッチング
+    let query = MirQueryBox::new(target_func);
+    let entry_id = target_func.entry_block;
+
+    // Check 1: Entry block has at least 1 successor
+    let entry_succs = query.succs(entry_id);
+    if entry_succs.is_empty() {
+        eprintln!("[joinir/skip_ws/mir] unexpected MIR shape: entry has no successors, falling back to handwritten");
+        return lower_skip_ws_handwritten(module);
+    }
+
+    // Check 2: First block contains Const(0) and BoxCall(String.length)
+    let entry_insts = query.insts_in_block(entry_id);
+    let has_const_0 = entry_insts.iter().any(|inst| {
+        matches!(inst, MirInstruction::Const { value: crate::mir::ConstValue::Integer(0), .. })
+    });
+    let has_string_length = entry_insts.iter().any(|inst| {
+        matches!(inst, MirInstruction::BoxCall { method, .. } if method == "length")
+    });
+
+    if !has_const_0 || !has_string_length {
+        eprintln!("[joinir/skip_ws/mir] unexpected MIR shape: entry block missing Const(0) or String.length, falling back to handwritten");
+        return lower_skip_ws_handwritten(module);
+    }
+
+    eprintln!("[joinir/skip_ws/mir] CFG sanity checks passed ✅");
+
     // JoinIR の ValueId は手書き版と同じレンジを使い、既存テストと互換にする
     let skip_id = JoinFuncId::new(0);
     let loop_step_id = JoinFuncId::new(1);