feat(Ring0): Phase 107 - Ring0.FsApi FileIo integration complete

Implementation Summary:
Phase 107 establishes FileBox as a unified layer over Ring0.FsApi

Completed Tasks:
- Task 1: FsApi SSOT review and documentation
- Task 2: FileIo designed as FsApi wrapper
- Task 3: Ring0FsFileIo implementation added
- Task 4: Fail-Fast integration verified
- Task 5: Documentation integration complete

Key Changes:
1. New File: src/providers/ring1/file/ring0_fs_fileio.rs
   - Ring0.FsApi-based FileIo implementation
   - Stateful wrapper (open/read/close)
   - UTF-8 handling via read_to_string()
   - One-file-at-a-time semantics (tested)

2. provider_lock.rs:
   - Added init_default_filebox_provider()
   - Auto-registration helper for Ring0FsFileIo

3. plugin_host.rs:
   - with_core_from_registry_optional() auto-registers default provider
   - Plugin priority maintained (debug logging)
   - Phase 106 MissingService check still active (Fail-Fast preserved)

4. Documentation:
   - phase107_fsapi_fileio_bridge.md: Complete design doc
   - phase106_filebox_design_revised.md: Phase 107 integration notes
   - core_boxes_design.md: Layer diagram and principles

Design Decisions:
- UTF-8 handling: read_to_string() for text-focused use cases
- One file at a time: open() returns Err if already open
- Plugin priority: init_default_filebox_provider() fails gracefully

Test Results:
- cargo build --release: SUCCESS
- plugin_host tests: 11 passed
- ring0_fs_fileio tests: 4 passed

Next Steps (Phase 108+):
- minimal/no-fs profile support
- write operations
- multi-file handle support

src/runtime/plugin_host.rs

@@ -155,8 +155,25 @@ impl PluginHost {
         use crate::runtime::core_services::*;
         use crate::runtime::provider_lock;
 
+        // Phase 107: Auto-register default FileBox provider (Ring0.FsApi-based)
+        // This happens before the Phase 106 check, so plugins can still override.
+        // If a plugin has already registered, this returns Err but we continue.
+        if CoreBoxId::File.is_core_required() {
+            match provider_lock::init_default_filebox_provider(&ring0) {
+                Ok(()) => {
+                    // Default provider registered successfully
+                    ring0.log.debug("[Phase 107] Ring0FsFileIo registered as default FileBox provider");
+                }
+                Err(msg) => {
+                    // Plugin provider already registered - this is OK (plugin priority)
+                    ring0.log.debug(&format!("[Phase 107] {}", msg));
+                }
+            }
+        }
+
         // Phase 106: FileBox provider チェック追加
         // CoreBoxId がFileを必須と判定している場合、provider が登録されていることを確認
+        // Phase 107: With auto-registration above, this check should always pass
         if CoreBoxId::File.is_core_required() {
             if provider_lock::get_filebox_provider().is_none() {
                 return Err(CoreInitError::MissingService {
@@ -303,15 +320,13 @@ mod tests {
         // Phase 94: 実際の registry を使用してテスト
         use crate::runtime::ring0::default_ring0;
         use crate::box_factory::builtin::BuiltinBoxFactory;
-        use crate::boxes::file::core_ro::CoreRoFileIo;
-        use crate::runtime::provider_lock;
 
         let ring0 = Arc::new(default_ring0());
         let mut registry = UnifiedBoxRegistry::new();
         registry.register(Arc::new(BuiltinBoxFactory::new()));
 
-        // Phase 106: Initialize FileBox provider before PluginHost creation
-        let _ = provider_lock::set_filebox_provider(Arc::new(CoreRoFileIo::new()));
+        // Phase 107: FileBox provider auto-registration (no manual setup needed)
+        // with_core_from_registry will call init_default_filebox_provider internally
 
         let plugin_host = PluginHost::with_core_from_registry(ring0, &registry)
             .expect("CoreServices should be initialized with builtin boxes");
@@ -325,15 +340,13 @@ mod tests {
         // Phase 94: 実際の registry を使用して全フィールドが存在することを確認
         use crate::runtime::ring0::default_ring0;
         use crate::box_factory::builtin::BuiltinBoxFactory;
-        use crate::boxes::file::core_ro::CoreRoFileIo;
-        use crate::runtime::provider_lock;
 
         let ring0 = Arc::new(default_ring0());
         let mut registry = UnifiedBoxRegistry::new();
         registry.register(Arc::new(BuiltinBoxFactory::new()));
 
-        // Phase 106: Initialize FileBox provider before PluginHost creation
-        let _ = provider_lock::set_filebox_provider(Arc::new(CoreRoFileIo::new()));
+        // Phase 107: FileBox provider auto-registration (no manual setup needed)
+        // with_core_from_registry will call init_default_filebox_provider internally
 
         let plugin_host = PluginHost::with_core_from_registry(ring0, &registry)
             .expect("CoreServices should be initialized");

src/runtime/provider_lock.rs

@@ -67,3 +67,28 @@ pub fn get_filebox_provider() -> Option<&'static Arc<dyn FileIo>> {
 pub fn get_filebox_caps() -> Option<FileCaps> {
     get_filebox_provider().map(|p| p.caps())
 }
+
+/// Phase 107: Initialize default FileBox provider using Ring0.FsApi
+///
+/// This helper registers Ring0FsFileIo as the default FileBox provider.
+/// It should be called during runtime initialization, after CoreServices setup.
+///
+/// # Returns
+///
+/// - `Ok(())`: Default provider registered successfully
+/// - `Err(msg)`: Provider already registered (plugin took precedence)
+///
+/// # Design
+///
+/// Plugin providers have priority over the default. If a plugin has already
+/// registered a FileBox provider, this function returns Err but the system
+/// continues normally (plugin priority is honored).
+pub fn init_default_filebox_provider(
+    ring0: &Arc<crate::runtime::ring0::Ring0Context>
+) -> Result<(), String> {
+    use crate::providers::ring1::file::ring0_fs_fileio::Ring0FsFileIo;
+
+    let provider = Arc::new(Ring0FsFileIo::new(ring0.clone()));
+    set_filebox_provider(provider)
+        .map_err(|_| "Plugin FileBox provider already registered".to_string())
+}