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/utils.rs

@@ -334,6 +334,7 @@ impl super::MirBuilder {
     }
 
     /// Ensure a value has a local definition in the current block by inserting a Copy.
+    #[allow(dead_code)]
     pub(crate) fn materialize_local(&mut self, v: super::ValueId) -> Result<super::ValueId, String> {
         // Phase 25.1b: Use function-local ID allocator to avoid SSA verification failures
         let dst = if let Some(ref mut f) = self.current_function {
@@ -348,6 +349,7 @@ impl super::MirBuilder {
     }
 
     /// Insert a Copy immediately after PHI nodes in the current block (position-stable).
+    #[allow(dead_code)]
     pub(crate) fn insert_copy_after_phis(&mut self, dst: super::ValueId, src: super::ValueId) -> Result<(), String> {
         if let (Some(ref mut function), Some(bb)) = (&mut self.current_function, self.current_block) {
             if std::env::var("NYASH_SCHEDULE_TRACE").ok().as_deref() == Some("1") {