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/funcscanner_trim.rs

@@ -50,7 +50,20 @@ use crate::mir::join_ir::{
     LoopExitShape, LoopHeaderShape, MirLikeInst,
 };
 
+/// Phase 27.9: Toggle dispatcher for trim lowering
+/// - Default: handwritten lowering
+/// - NYASH_JOINIR_LOWER_FROM_MIR=1: MIR-based lowering
 pub fn lower_funcscanner_trim_to_joinir(module: &crate::mir::MirModule) -> Option<JoinModule> {
+    super::common::dispatch_lowering(
+        "trim",
+        module,
+        lower_trim_from_mir,
+        lower_trim_handwritten,
+    )
+}
+
+/// Phase 27.1-27.7: Handwritten JoinIR lowering for FuncScannerBox.trim/1
+fn lower_trim_handwritten(module: &crate::mir::MirModule) -> Option<JoinModule> {
     // Step 1: "FuncScannerBox.trim/1" を探す
     let target_func = module.functions.get("FuncScannerBox.trim/1")?;
 
@@ -495,3 +508,47 @@ pub fn lower_funcscanner_trim_to_joinir(module: &crate::mir::MirModule) -> Optio
 
     Some(join_module)
 }
+
+/// Phase 27.9: MIR-based lowering for FuncScannerBox.trim/1
+/// - Lightweight CFG sanity checks
+/// - Fallback to handwritten if MIR structure is unexpected
+fn lower_trim_from_mir(module: &crate::mir::MirModule) -> Option<JoinModule> {
+    use crate::mir::query::MirQueryBox;
+    use crate::mir::BinaryOp;
+    use super::common::{ensure_entry_has_succs, has_const_string, has_string_method, has_binop, log_fallback};
+
+    // Step 1: "FuncScannerBox.trim/1" を探す
+    let target_func = module.functions.get("FuncScannerBox.trim/1")?;
+
+    eprintln!("[joinir/trim/mir] Found FuncScannerBox.trim/1 (MIR-based lowering)");
+    eprintln!("[joinir/trim/mir] MIR blocks: {}", target_func.blocks.len());
+
+    // 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
+    if !ensure_entry_has_succs(&query, entry_id) {
+        log_fallback("trim", "entry has no successors");
+        return lower_trim_handwritten(module);
+    }
+
+    // Check 2: Entry block contains expected patterns
+    // - Const("") for string coercion
+    // - BoxCall(String.length)
+    // - BinOp(Add) for "" + s
+    if !has_const_string(&query, entry_id, "") ||
+       !has_string_method(&query, entry_id, "length") ||
+       !has_binop(&query, entry_id, BinaryOp::Add)
+    {
+        log_fallback("trim", "entry block missing expected patterns (Const(\"\"), String.length, or BinOp(Add))");
+        return lower_trim_handwritten(module);
+    }
+
+    eprintln!("[joinir/trim/mir] CFG sanity checks passed ✅");
+
+    // Phase 27.9: Generate JoinIR (equivalent to handwritten version)
+    // For now, use the same structure as handwritten version
+    // TODO: Full MIR-based construction in future phases
+    lower_trim_handwritten(module)
+}