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/boxes/file/mod.rs

@@ -87,19 +87,19 @@ impl FileBox {
         }
     }
 
-    pub fn write_all(&self, _buf: &[u8]) -> Result<(), String> {
-        // Fail-Fast by capability: consult provider caps
-        let caps = self
-            .provider
-            .as_ref()
-            .map(|p| p.caps())
-            .or_else(|| provider_lock::get_filebox_caps())
-            .unwrap_or_else(|| provider::FileCaps::read_only());
-        if !caps.write {
-            return Err("Write unsupported by current FileBox provider (read-only)".to_string());
+    pub fn write_all(&self, buf: &[u8]) -> Result<(), String> {
+        if let Some(ref provider) = self.provider {
+            let caps = provider.caps();
+            if !caps.write {
+                return Err("Write not supported by FileBox provider".to_string());
+            }
+            // Phase 108: UTF-8 conversion (text-oriented design)
+            let text = String::from_utf8_lossy(buf).to_string();
+            provider.write(&text)
+                .map_err(|e| format!("Write failed: {:?}", e))
+        } else {
+            Err("No provider available".to_string())
         }
-        // Write-capable provider not wired yet
-        Err("Write supported by provider but not implemented in this build".to_string())
     }
 
     /// ファイルの内容を読み取る
@@ -111,21 +111,28 @@ impl FileBox {
     }
 
     /// ファイルに内容を書き込む
-    pub fn write(&self, _content: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
-        let caps = self
-            .provider
-            .as_ref()
-            .map(|p| p.caps())
-            .or_else(|| provider_lock::get_filebox_caps())
-            .unwrap_or_else(|| provider::FileCaps::read_only());
-        if !caps.write {
-            return Box::new(StringBox::new(
-                "Error: write unsupported by provider (read-only)",
-            ));
+    pub fn write(&self, content: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
+        if let Some(ref provider) = self.provider {
+            let caps = provider.caps();
+            if !caps.write {
+                return Box::new(StringBox::new(
+                    "Error: write not supported by provider (read-only)".to_string()
+                ));
+            }
+            // Phase 108: Convert content to text
+            let text = if let Some(str_box) = content.as_any().downcast_ref::<StringBox>() {
+                str_box.to_string_box().value
+            } else {
+                content.to_string_box().value
+            };
+
+            match provider.write(&text) {
+                Ok(()) => Box::new(StringBox::new("OK".to_string())),
+                Err(e) => Box::new(StringBox::new(format!("Error: {:?}", e))),
+            }
+        } else {
+            Box::new(StringBox::new("Error: no provider available".to_string()))
         }
-        Box::new(StringBox::new(
-            "Error: write supported but not implemented in this build",
-        ))
     }
 
     /// ファイルが存在するかチェック

src/boxes/file/provider.rs

@@ -52,8 +52,9 @@ pub trait FileIo: Send + Sync {
     fn caps(&self) -> FileCaps;
     fn open(&self, path: &str) -> FileResult<()>;
     fn read(&self) -> FileResult<String>;
+    fn write(&self, text: &str) -> FileResult<()>; // Phase 108: write support
     fn close(&self) -> FileResult<()>;
-    // Future: write/exists/stat …
+    // Future: exists/stat …
 }
 
 /// Normalize newlines to LF (optional helper)