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/host_providers/mir_builder.rs

@@ -2,7 +2,7 @@ use crate::runner;
 use serde_json::Value as JsonValue;
 use std::collections::HashMap;
 use std::fs;
-use std::io::Write;
+// use std::io::Write; // kept for future pretty-print extensions
 
 /// Convert Program(JSON v0) to MIR(JSON v0) and return it as a String.
 /// Fail-Fast: prints stable tags on stderr and returns Err with the same tag text.
@@ -52,7 +52,7 @@ pub fn program_json_to_mir_json_with_imports(program_json: &str, imports: HashMa
             let _ = fs::remove_file(&tmp_path);
             // Ensure v1 schema: if output lacks schema_version but has functions, wrap minimally
             let s = match serde_json::from_str::<JsonValue>(&s0) {
-                Ok(JsonValue::Object(mut m)) => {
+                Ok(JsonValue::Object(m)) => {
                     if m.get("schema_version").is_none() {
                         if let Some(funcs) = m.get("functions").cloned() {
                             let v1 = serde_json::json!({"schema_version":"1.0","functions": funcs});