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>

src/runtime/plugin_host.rs

@@ -423,37 +423,31 @@ mod tests {
     }
 
     #[test]
-    fn test_with_core_from_registry_filebox_required() {
-        // Phase 106: FileBox provider なし → エラー
-        // Note: この test は provider_lock::get_filebox_provider() が None を返す場合のみ有効。
-        // OnceLock の性質上、同一プロセス内で他のテストが先に provider を set すると
-        // このテストは期待通りに動作しない（provider が既に存在するため）。
-        // そのため、provider が既に set されている場合は test を skip する。
+    fn test_with_core_from_registry_filebox_auto_registered() {
+        // Phase 107/108: with_core_from_registry() は Ring0FsFileIo を自動登録するため、
+        // FileBox は常に利用可能になる
 
         use crate::runtime::ring0::default_ring0;
         use crate::box_factory::builtin::BuiltinBoxFactory;
-        use crate::runtime::provider_lock;
-
-        // provider が既に set されている場合は test skip
-        if provider_lock::get_filebox_provider().is_some() {
-            eprintln!("Skipping test_with_core_from_registry_filebox_required: provider already set by another test");
-            return;
-        }
 
         let ring0 = Arc::new(default_ring0());
         let mut registry = UnifiedBoxRegistry::new();
         registry.register(Arc::new(BuiltinBoxFactory::new()));
 
-        // provider_lock を初期化せず（呼び出さず）
-        // provider が無い状態で with_core_from_registry() を呼ぶ
-
+        // Phase 107: with_core_from_registry() は Ring0FsFileIo を自動登録
         let result = PluginHost::with_core_from_registry(ring0, &registry);
-        assert!(result.is_err());
 
-        if let Err(CoreInitError::MissingService { box_id, .. }) = result {
-            assert_eq!(box_id, CoreBoxId::File);
+        // Phase 107/108: FileBox provider は自動登録されるため、成功するはず
+        assert!(result.is_ok(), "Expected success with auto-registered FileBox provider");
+
+        // Phase 108: 登録された provider は read/write 両対応
+        use crate::runtime::provider_lock;
+        if let Some(provider) = provider_lock::get_filebox_provider() {
+            let caps = provider.caps();
+            assert!(caps.read, "FileBox provider should support read");
+            assert!(caps.write, "FileBox provider should support write (Phase 108)");
         } else {
-            panic!("Expected MissingService error for FileBox");
+            panic!("FileBox provider should be registered after with_core_from_registry");
         }
     }
 }