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/lifecycle.rs

@@ -288,7 +288,7 @@ impl super::MirBuilder {
                                     MirInstruction::Const { value, .. } => {
                                         if let super::ConstValue::String(s) = value { last_const_name = Some(s.clone()); }
                                     }
-                                    MirInstruction::Call { func, .. } => {
+                                    MirInstruction::Call { func: _, .. } => {
                                         // If immediately preceded by matching Const String, accept
                                         if let Some(prev) = last_const_name.as_ref() {
                                             if prev == &expect_tail { ok = true; break; }
@@ -318,7 +318,7 @@ impl super::MirBuilder {
         // Dev stub: provide condition_fn when missing to satisfy predicate calls in JSON lexers
         // Returns integer 1 (truthy) and accepts one argument (unused).
         if module.functions.get("condition_fn").is_none() {
-            let mut sig = FunctionSignature {
+            let sig = FunctionSignature {
                 name: "condition_fn".to_string(),
                 params: vec![MirType::Integer], // accept one i64-like arg
                 return_type: MirType::Integer,