refactor(joinir): Phase 27.10 - CFG sanity checks + dispatcher pattern 共通化

- common.rs 新規作成（162行）:
  - CFG sanity check helpers: ensure_entry_has_succs, has_const_int, has_const_string, has_string_method, has_binop
  - Logging helper: log_fallback
  - Dispatcher: dispatch_lowering
- skip_ws.rs: CFG checks (-25行) + dispatcher (-2行) = -27行削減
- funcscanner_trim.rs: CFG checks (-25行) + dispatcher (-4行) = -29行削減
- mod.rs: pub mod common 追加

設計原則:
- 軽量パターンマッチング（命令の存在確認のみ）
- Graceful degradation（予期しない構造で即座にfallback）
- DRY原則（重複コード1箇所に集約）

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

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

src/mir/join_ir/lowering/skip_ws.rs

@@ -41,7 +41,7 @@
 
 use crate::mir::ValueId;
 use crate::mir::join_ir::{
-    env_flag_is_1, BinOpKind, CompareOp, ConstValue, JoinContId, JoinFuncId, JoinFunction,
+    BinOpKind, CompareOp, ConstValue, JoinContId, JoinFuncId, JoinFunction,
     JoinInst, JoinModule, LoopExitShape, LoopHeaderShape, MirLikeInst,
 };
 
@@ -229,8 +229,8 @@ 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;
+    use crate::mir::query::MirQueryBox;
+    use super::common::{ensure_entry_has_succs, has_const_int, has_string_method, log_fallback};
 
     // Step 1: "Main.skip/1" を探す
     let target_func = module.functions.get("Main.skip/1")?;
@@ -245,33 +245,23 @@ fn lower_skip_ws_from_mir(module: &crate::mir::MirModule) -> Option<JoinModule>
 
     // 簡易チェック: ブロック数が最低限あるか確認
     if target_func.blocks.len() < 3 {
-        eprintln!("[joinir/skip_ws/mir] insufficient blocks ({}), falling back", target_func.blocks.len());
+        log_fallback("skip_ws", &format!("insufficient blocks ({})", target_func.blocks.len()));
         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
+    // Phase 27.10: Lightweight CFG sanity checks using common utilities
     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");
+    if !ensure_entry_has_succs(&query, entry_id) {
+        log_fallback("skip_ws", "entry has no successors");
         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");
+    // Check 2: Entry block contains Const(0) and BoxCall(String.length)
+    if !has_const_int(&query, entry_id, 0) || !has_string_method(&query, entry_id, "length") {
+        log_fallback("skip_ws", "entry block missing Const(0) or String.length");
         return lower_skip_ws_handwritten(module);
     }
 
@@ -435,11 +425,10 @@ fn lower_skip_ws_from_mir(module: &crate::mir::MirModule) -> Option<JoinModule>
 /// NYASH_JOINIR_LOWER_FROM_MIR=1 ./target/release/hakorune program.hako
 /// ```
 pub fn lower_skip_ws_to_joinir(module: &crate::mir::MirModule) -> Option<JoinModule> {
-    if env_flag_is_1("NYASH_JOINIR_LOWER_FROM_MIR") {
-        eprintln!("[joinir/skip_ws] Using MIR-based lowering (NYASH_JOINIR_LOWER_FROM_MIR=1)");
-        lower_skip_ws_from_mir(module)
-    } else {
-        eprintln!("[joinir/skip_ws] Using handwritten lowering (default)");
-        lower_skip_ws_handwritten(module)
-    }
+    super::common::dispatch_lowering(
+        "skip_ws",
+        module,
+        lower_skip_ws_from_mir,
+        lower_skip_ws_handwritten,
+    )
 }