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/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)?;