Phase 25.1b: Step2完了（FuncBodyBasicLowerBox導入）

Step2実装内容:
- FuncBodyBasicLowerBox導入（defs専用下請けモジュール）
- _try_lower_local_if_return実装（Local+単純if）
- _inline_local_ints実装（軽い正規化）
- minimal lowers統合（Return/BinOp/IfCompare/MethodArray系）

Fail-Fast体制確立:
- MirBuilderBox: defs_onlyでも必ずタグ出力
- [builder/selfhost-first:unsupported:defs_only]
- [builder/selfhost-first:unsupported:no_match]

Phase構造整備:
- Phase 25.1b README新設（Step0-3計画）
- Phase 25.2b README新設（次期計画）
- UsingResolverBox追加（using system対応準備）

スモークテスト:
- stage1_launcher_program_to_mir_canary_vm.sh追加

Next: Step3 LoopForm対応

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

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

src/runner/modes/vm.rs

@@ -155,6 +155,34 @@ impl NyashRunner {
                 crate::runner::modes::common_util::hako::strip_local_decl(&code_final);
         }
 
+        // Optional: dump merged Hako source after using/prelude merge and Hako normalization.
+        // Guarded by env; defaultはOFF（Phase 25.1a selfhost builder デバッグ用）。
+        if std::env::var("NYASH_VM_DUMP_MERGED_HAKO")
+            .ok()
+            .as_deref()
+            == Some("1")
+        {
+            let default_path = {
+                let mut tmp = std::env::temp_dir();
+                tmp.push("nyash_merged_vm.hako");
+                tmp
+            };
+            let path = std::env::var("NYASH_VM_DUMP_MERGED_HAKO_PATH")
+                .ok()
+                .filter(|p| !p.is_empty())
+                .unwrap_or_else(|| default_path.to_string_lossy().into_owned());
+
+            if let Err(e) = fs::write(&path, &code_final) {
+                if trace {
+                    eprintln!("[vm/merged-hako] failed to write {}: {}", path, e);
+                }
+            } else if trace
+                || crate::config::env::env_bool("NYASH_VM_DUMP_MERGED_HAKO_LOG")
+            {
+                eprintln!("[vm/merged-hako] dumped merged code to {}", path);
+            }
+        }
+
         if trace && (std::env::var("NYASH_PARSER_STAGE3").ok() == Some("1".into())
             || std::env::var("HAKO_PARSER_STAGE3").ok() == Some("1".into()))
         {