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/phi_core/if_phi.rs

@@ -145,7 +145,7 @@ pub fn merge_modified_at_merge_with<O: PhiMergeOps>(
     ops: &mut O,
     merge_bb: crate::mir::BasicBlockId,
     _then_block: crate::mir::BasicBlockId,
-    else_block: crate::mir::BasicBlockId,
+    _else_block: crate::mir::BasicBlockId,
     then_pred_opt: Option<crate::mir::BasicBlockId>,
     else_pred_opt: Option<crate::mir::BasicBlockId>,
     pre_if_snapshot: &HashMap<String, ValueId>,

src/mir/phi_core/loop_phi.rs

@@ -193,7 +193,7 @@ pub fn prepare_loop_variables_with<O: LoopPhiOps>(
         ops.emit_copy_at_preheader(preheader_id, pre_copy, value_before)?;
 
         let phi_id = ops.new_value();
-        let mut inc = IncompletePhi {
+        let inc = IncompletePhi {
             phi_id,
             var_name: var_name.clone(),
             known_inputs: vec![(preheader_id, pre_copy)],  // ensure def at preheader