feat(mir): Phase 25.1f完了 - Conservative PHI + ControlForm観測レイヤー

🎉 Conservative PHI Box理論による完全SSA構築

**Phase 7-B: Conservative PHI実装**
- 片方branchのみ定義変数に対応（emit_void使用）
- 全変数にPHI生成（Conservative Box理論）
- Stage-1 resolver全テスト緑化（3/3 PASS）

**Phase 25.1f: ControlForm観測レイヤー**
- LoopShape/IfShape/ControlForm構造定義
- Loop/If統一インターフェース実装
- debug_dump/debug_validate機能追加
- NYASH_CONTROL_FORM_TRACE環境変数対応

**主な変更**:
- src/mir/builder/phi.rs: Conservative PHI実装
- src/mir/control_form.rs: ControlForm構造（NEW）
- src/mir/loop_builder.rs: LoopForm v2デフォルト化

**テスト結果**:
✅ mir_stage1_using_resolver_min_fragment_verifies
✅ mir_stage1_using_resolver_full_collect_entries_verifies
✅ mir_parserbox_parse_program2_harness_parses_minimal_source

🤖 Generated with Claude Code
Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: ChatGPT <chatgpt@openai.com>

src/parser/statements/helpers.rs

@@ -59,6 +59,7 @@ impl NyashParser {
     }
 
     /// Small helper: build UnexpectedToken with current token and line
+    #[allow(dead_code)]
     pub(super) fn err_unexpected<S: Into<String>>(&self, expected: S) -> ParseError {
         ParseError::UnexpectedToken {
             found: self.current_token().token_type.clone(),
@@ -68,6 +69,7 @@ impl NyashParser {
     }
 
     /// Expect an identifier and advance. Returns its string or an UnexpectedToken error
+    #[allow(dead_code)]
     pub(super) fn expect_identifier(&mut self, what: &str) -> Result<String, ParseError> {
         if let TokenType::IDENTIFIER(name) = &self.current_token().token_type {
             let out = name.clone();
@@ -77,4 +79,4 @@ impl NyashParser {
             Err(self.err_unexpected(what))
         }
     }
-}
+}

src/parser/statements/mod.rs

@@ -131,7 +131,7 @@ impl NyashParser {
         let mut statements = Vec::new();
 
         // Helper: lookahead for `ident '(' ... ')' [NEWLINE*] '{'`
-        let mut looks_like_method_head = |this: &Self| -> bool {
+        let looks_like_method_head = |this: &Self| -> bool {
             // Only meaningful when starting at a new statement head
             match &this.current_token().token_type {
                 TokenType::IDENTIFIER(_) => {