feat(filebox): Phase 108 FileBox write/write_all implementation

Implement write functionality for FileBox following Phase 108 specification,
completing the read/write pipeline via Ring0.FsApi.

## Implementation Summary

### Task 2: FsApi / Ring0FsFileIo write implementation
- Added write() method to FileIo trait
- Implemented write() in Ring0FsFileIo (truncate mode via Ring0.FsApi.write_all)
- Updated FileCaps to { read: true, write: true } for standard profile
- Added write() stub to CoreRoFileIo (returns Unsupported)

### Task 3: FileBox write/write_all implementation
- Updated FileBox.write_all() to delegate to provider.write()
- Updated FileBox.write() to convert content to text and call provider.write()
- UTF-8 conversion via String::from_utf8_lossy (text-oriented design)
- Returns "OK" on success, "Error: ..." on failure

### Task 4: Test coverage
- Round-trip test (write → read): ✅ PASS
- Truncate mode verification: ✅ PASS
- Write without open error: ✅ PASS
- Read-only provider rejection: ✅ PASS
- Auto-registration test updated: ✅ PASS

### Task 5: Documentation updates
- phase107_fsapi_fileio_bridge.md: Added Phase 108 section
- core_boxes_design.md: Updated Ring0.FsApi relationship section
- CURRENT_TASK.md: Added Phase 108 completion entry

## Design Decisions (from phase108_filebox_write_semantics.md)

- **Write mode**: truncate (overwrite existing file each time)
- **Text-oriented**: UTF-8 conversion via from_utf8_lossy
- **Append mode**: Planned for Phase 109+
- **Error handling**: FileError::Io for failures, Fail-Fast on caps.write=false

## Test Results

```
cargo test --release --lib filebox
test result: ok. 5 passed; 0 failed; 1 ignored
```

All FileBox tests pass, including Phase 107 compatibility tests.

## Pipeline Complete

```
FileBox.write(content)
    ↓
FileBox.write_all(buf)
    ↓
provider.write(text) ← Ring0FsFileIo implementation
    ↓
Ring0.FsApi.write_all()
    ↓
std::fs::write()
```

## Next Steps (Backlog)

- Phase 109: minimal/no-fs profile
- Phase 110: FileHandleBox (multiple files simultaneously)
- Phase 111: append mode implementation

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

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

docs/development/current/main/core_boxes_design.md

@@ -256,7 +256,7 @@ FileBox は selfhost/通常ランタイムでは事実上必須（ログ・ツ
 - **PluginHost**: startup 時に CoreBoxId.is_core_required() で provider をチェック
   - 未登録なら CoreInitError::MissingService で fail-fast
 
-### Ring0.FsApi との関係（Phase 107 完了）✅
+### Ring0.FsApi との関係（Phase 107-108 完了）✅
 
 **Phase 107 統合完了（2025-12-03）**:
 
@@ -266,16 +266,22 @@ FileBox の実体 I/O は、以下の層構造で Ring0.FsApi を通す設計が
 [FileBox (Ring1)]
     ↓ provider 経由
 [Ring0FsFileIo] (FileIo 実装)
-    ↓ read_to_string/read 呼び出し
+    ↓ read_to_string/write_all 呼び出し
 [Ring0.FsApi] (OS I/O 抽象)
     ↓
 [std::fs]
 ```
 
+**Phase 108 実装完了（2025-12-03）**:
+- FileBox は Ring0FsFileIo 経由で **read/write 両対応**
+- write は **truncate mode**（毎回上書き）
+- append モードは Phase 109+ で予定
+
 **設計原則**:
 - **FileIo = stateful**（現在開いているファイルハンドルに対する操作）
   - open() でファイルを開く
   - read() で内容を読み込む
+  - write() で内容を書き込む（Phase 108 追加）
   - close() でファイルを閉じる
 - **FsApi = stateless**（Path → データの直接変換）
   - read_to_string(path) / write_all(path, data)

docs/development/current/main/phase107_fsapi_fileio_bridge.md

@@ -316,7 +316,28 @@
 
 ---
 
-## 8. 設計原則（Phase 107 で確立）
+## 8. Phase 108 以降の進展
+
+### Phase 108 実装完了（2025-12-03）
+
+**write 実装完了**:
+- Ring0FsFileIo に write() メソッド実装
+- FileBox.write() / write_all() が Ring0.FsApi.write_all() 経由で動作
+- FileCaps.write = true（標準プロファイルで書き込み対応）
+
+**設計決定**:
+- **Write mode**: truncate（既存ファイル毎回上書き）
+- **テキスト前提**: UTF-8 変換（from_utf8_lossy）
+- **append mode**: Phase 109+ で予定
+
+**テスト**:
+- Round-trip テスト（write → read）✅
+- Truncate mode 検証✅
+- Read-only provider 拒否テスト✅
+
+---
+
+## 9. 設計原則（Phase 107 で確立）
 
 ### 層の棲み分けが完全化