fix(mir): PHI検証panic修正 - update_cfg()を検証前に呼び出し

A案実装: debug_verify_phi_inputs呼び出し前にCFG predecessorを更新

修正箇所（7箇所）:
- src/mir/builder/phi.rs:50, 73, 132, 143
- src/mir/builder/ops.rs:273, 328, 351

根本原因:
- Branch/Jump命令でsuccessorは即座に更新
- predecessorはupdate_cfg()で遅延再構築
- PHI検証が先に実行されてpredecessor未更新でpanic

解決策:
- 各debug_verify_phi_inputs呼び出し前に
  if let Some(func) = self.current_function.as_mut() {
      func.update_cfg();
  }
  を挿入してCFGを同期

影響: if/else文、論理演算子(&&/||)のPHI生成が正常動作

src/mir/builder/decls.rs

@@ -54,6 +54,15 @@ impl super::MirBuilder {
                         self
                             .value_types
                             .insert(pid, super::MirType::Box("ArrayBox".to_string()));
+                        // Explicitly call birth() to initialize internal state
+                        self.emit_instruction(MirInstruction::BoxCall {
+                            dst: None,
+                            box_val: pid,
+                            method: "birth".to_string(),
+                            method_id: None,
+                            args: vec![],
+                            effects: super::EffectMask::MUT,
+                        })?;
                         if let Some(args) = script_args.as_ref() {
                             for arg in args {
                                 let val = crate::mir::builder::emission::constant::emit_string(self, arg.clone());
@@ -75,7 +84,8 @@ impl super::MirBuilder {
                     }
                     self.variable_map.insert(p.clone(), pid);
                 }
-                let lowered = self.build_expression(program_ast);
+                // Lower statements in order to preserve def→use
+                let lowered = self.cf_block(body.clone());
                 self.variable_map = saved_var_map;
                 lowered
             } else {