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/mir/builder/decls.rs

@@ -38,11 +38,6 @@ impl super::MirBuilder {
                     let _ = self.lower_static_method_as_function(func_name, params.clone(), body.clone());
                     eprintln!("[DEBUG] build_static_main_box: After lower_static_method_as_function");
                     eprintln!("[DEBUG]   variable_map = {:?}", self.variable_map);
-                    // Convert the method body to a Program AST node and lower it
-                    let program_ast = ASTNode::Program {
-                    statements: body.clone(),
-                    span: crate::ast::Span::unknown(),
-                };
                 // Initialize local variables for Main.main() parameters
                 // Note: These are local variables in the wrapper main() function, NOT parameters
                 let saved_var_map = std::mem::take(&mut self.variable_map);
@@ -117,7 +112,7 @@ impl super::MirBuilder {
         weak_fields: Vec<String>,
     ) -> Result<(), String> {
         // Create a type registration constant (marker)
-        let type_id = crate::mir::builder::emission::constant::emit_string(self, format!("__box_type_{}", name));
+        crate::mir::builder::emission::constant::emit_string(self, format!("__box_type_{}", name));
 
         // Emit field metadata markers
         for field in fields {