feat(phase89-a): Ring0Context logs migration - 56 locations

Phase 89-A 完了: println!/eprintln! → ring0.log.* 移行

**実装内容**:
- 環境変数制御: NYASH_RING0_LOG_LEVEL (DEBUG/INFO/WARN/ERROR)
- selfhost.rs: 24箇所移行 (error:14, warn:2, info:5, debug:3)
- vm.rs: 32箇所移行 (error:5, warn:4, info:2, debug:21)

**実装効果**:
- 総移行箇所: 56箇所 (目標: 10-15箇所 → 373%達成)
- 累計進捗: 58/3,955箇所 (1.47%)
- テスト結果: 521 passed; 33 failed (変化なし)

**次のステップ**: Phase 89-B (IO/time 棚卸し)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/runtime/ring0/std_impls.rs

@@ -61,11 +61,39 @@ impl TimeApi for StdTime {
 /// eprintln!/println! ベースのログ実装
 pub struct StdLog;
 
+impl StdLog {
+    fn should_log(&self, level: LogLevel) -> bool {
+        let min_level_str = std::env::var("NYASH_RING0_LOG_LEVEL")
+            .unwrap_or_else(|_| "INFO".to_string());
+
+        let min_level = match min_level_str.to_uppercase().as_str() {
+            "DEBUG" => LogLevel::Debug,
+            "INFO" => LogLevel::Info,
+            "WARN" => LogLevel::Warn,
+            "ERROR" => LogLevel::Error,
+            _ => LogLevel::Info,
+        };
+
+        // level の優先度が min_level 以上なら true
+        matches!(
+            (level, min_level),
+            (LogLevel::Error, _) |
+            (LogLevel::Warn, LogLevel::Debug | LogLevel::Info | LogLevel::Warn) |
+            (LogLevel::Info, LogLevel::Debug | LogLevel::Info) |
+            (LogLevel::Debug, LogLevel::Debug)
+        )
+    }
+}
+
 impl LogApi for StdLog {
     fn log(&self, level: LogLevel, msg: &str) {
+        if !self.should_log(level) {
+            return;
+        }
+
         match level {
             LogLevel::Debug => eprintln!("[DEBUG] {}", msg),
-            LogLevel::Info => println!("[INFO] {}", msg),
+            LogLevel::Info => eprintln!("[INFO] {}", msg),
             LogLevel::Warn => eprintln!("[WARN] {}", msg),
             LogLevel::Error => eprintln!("[ERROR] {}", msg),
         }