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/json_v0_bridge/ast.rs

@@ -86,6 +86,7 @@ pub(super) enum ExprV0 {
     Bool {
         value: bool,
     },
+    Null,
     Binary {
         op: String,
         lhs: Box<ExprV0>,

src/runner/json_v0_bridge/lowering/expr.rs

@@ -65,6 +65,21 @@ impl<'a> VarScope for MapVars<'a> {
             self.vars.insert(name.to_string(), dst);
             return Ok(Some(dst));
         }
+        // Phase 25.1a: Treat `hostbridge` as a well-known global for bridge lowering.
+        // The actual extern dispatch is handled at runtime; here we only need a stable
+        // placeholder value so that Program(JSON) containing hostbridge.extern_invoke(...)
+        // can be lowered without "undefined variable" errors.
+        if name == "hostbridge" {
+            let dst = f.next_value_id();
+            if let Some(bb) = f.get_block_mut(cur_bb) {
+                bb.add_instruction(MirInstruction::Const {
+                    dst,
+                    value: ConstValue::String("hostbridge".into()),
+                });
+            }
+            self.vars.insert(name.to_string(), dst);
+            return Ok(Some(dst));
+        }
         if name == "me" {
             if env.allow_me_dummy {
                 let dst = f.next_value_id();
@@ -163,6 +178,16 @@ pub(super) fn lower_expr_with_scope<S: VarScope>(
             }
             Ok((dst, cur_bb))
         }
+        ExprV0::Null => {
+            let dst = f.next_value_id();
+            if let Some(bb) = f.get_block_mut(cur_bb) {
+                bb.add_instruction(MirInstruction::Const {
+                    dst,
+                    value: ConstValue::Null,
+                });
+            }
+            Ok((dst, cur_bb))
+        }
         ExprV0::Binary { op, lhs, rhs } => {
             let (l, cur_after_l) = lower_expr_with_scope(env, f, cur_bb, lhs, vars)?;
             let (r, cur_after_r) = lower_expr_with_scope(env, f, cur_after_l, rhs, vars)?;

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()))
         {