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/runner/modes/vm_fallback.rs

@@ -321,7 +321,7 @@ impl NyashRunner {
         }
         match vm.execute_module(&module_vm) {
             Ok(ret) => {
-                use crate::box_trait::{NyashBox, IntegerBox, BoolBox};
+                use crate::box_trait::{IntegerBox, BoolBox};
 
                 // Extract exit code from return value
                 let exit_code = if let Some(ib) = ret.as_any().downcast_ref::<IntegerBox>() {
@@ -346,7 +346,8 @@ impl NyashRunner {
 
 impl NyashRunner {
     /// Small helper to continue fallback execution once AST is prepared
-    fn execute_vm_fallback_from_ast(&self, filename: &str, ast: nyash_rust::ast::ASTNode) {
+    #[allow(dead_code)]
+    fn execute_vm_fallback_from_ast(&self, _filename: &str, ast: nyash_rust::ast::ASTNode) {
         use crate::{
             backend::MirInterpreter,
             box_factory::{BoxFactory, RuntimeError},