- write mode: truncate(既存ファイル毎回上書き)採用 - FileIo trait に write() を追加 - テキスト前提(from_utf8_lossy),append は Phase 109+ - FileIo trait 拡張時の互換性(DummyFileIo stub)を明記 - テスト設計:round-trip, read-only拒否, double-open確認 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>