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/parser/mod.rs

@@ -106,6 +106,9 @@ pub enum ParseError {
     #[error("Invalid statement at line {line}")]
     InvalidStatement { line: usize },
 
+    #[error("Unsupported identifier '{name}' at line {line}")]
+    UnsupportedIdentifier { name: String, line: usize },
+
     #[error("Circular dependency detected between static boxes: {cycle}")]
     CircularDependency { cycle: String },
 
@@ -215,6 +218,17 @@ impl NyashParser {
         let mut tokenizer = crate::tokenizer::NyashTokenizer::new(pre);
         let tokens = tokenizer.tokenize()?;
 
+        for tok in &tokens {
+            if let TokenType::IDENTIFIER(name) = &tok.token_type {
+                if name == "self" {
+                    return Err(ParseError::UnsupportedIdentifier {
+                        name: name.clone(),
+                        line: tok.line,
+                    });
+                }
+            }
+        }
+
         let mut parser = Self::new(tokens);
         parser.debug_fuel = fuel;
         let result = parser.parse();