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/schedule/block.rs

@@ -7,6 +7,7 @@ pub struct BlockScheduleBox;
 
 impl BlockScheduleBox {
     /// Insert a Copy immediately after PHI nodes. Returns the local value id.
+    #[allow(dead_code)]
     pub fn ensure_after_phis_copy(builder: &mut MirBuilder, src: ValueId) -> Result<ValueId, String> {
         if let Some(bb) = builder.current_block {
             if let Some(&cached) = builder.schedule_mat_map.get(&(bb, src)) {
@@ -29,6 +30,7 @@ impl BlockScheduleBox {
 
     /// Emit a Copy right before the next emitted instruction (best-effort):
     /// place it at the tail of the current block. Returns the local value id.
+    #[allow(dead_code)]
     pub fn emit_before_call_copy(builder: &mut MirBuilder, src: ValueId) -> Result<ValueId, String> {
         // Prefer to reuse the after-phis materialized id for this src in this block
         let base = Self::ensure_after_phis_copy(builder, src)?;