feat: Phase 1完了 - plugin_loader_v2大規模リファクタリング（1006→893行、420行分離）

✨ Single Responsibility Principle適用による構造改善
- extern_functions.rs: env.*外部関数処理（261行）
- ffi_bridge.rs: FFI/TLV処理（158行）
- instance_manager.rs: インスタンス管理（140行）
- loader.rs: 1006→893行（11%削減）

🎯 効果:
- 6つの責任を分離し単一責任原則遵守
- モジュール間の境界明確化
- 保守性・可読性大幅向上

📦 追加: filebox-pluginモジュール化も含む

plugins/nyash-filebox-plugin/src/state.rs

@@ -0,0 +1,92 @@
+//! State management for FileBox plugin
+
+use once_cell::sync::Lazy;
+use std::collections::HashMap;
+use std::sync::{
+    atomic::{AtomicU32, Ordering},
+    Mutex,
+};
+
+// ============ FileBox Instance ============
+pub struct FileBoxInstance {
+    pub file: Option<std::fs::File>,
+    pub path: String,
+    pub buffer: Option<Vec<u8>>, // プラグインが管理するバッファ
+}
+
+impl FileBoxInstance {
+    pub fn new() -> Self {
+        Self {
+            file: None,
+            path: String::new(),
+            buffer: None,
+        }
+    }
+
+    pub fn with_path(path: String) -> Self {
+        Self {
+            file: None,
+            path,
+            buffer: None,
+        }
+    }
+}
+
+// グローバルインスタンス管理
+pub static INSTANCES: Lazy<Mutex<HashMap<u32, FileBoxInstance>>> =
+    Lazy::new(|| Mutex::new(HashMap::new()));
+
+// インスタンスIDカウンタ（1開始）
+pub static INSTANCE_COUNTER: AtomicU32 = AtomicU32::new(1);
+
+/// Allocate a new instance ID
+pub fn allocate_instance_id() -> u32 {
+    INSTANCE_COUNTER.fetch_add(1, Ordering::Relaxed)
+}
+
+/// Store an instance with the given ID
+pub fn store_instance(id: u32, instance: FileBoxInstance) -> Result<(), &'static str> {
+    match INSTANCES.lock() {
+        Ok(mut map) => {
+            map.insert(id, instance);
+            Ok(())
+        }
+        Err(_) => Err("Failed to lock instances map"),
+    }
+}
+
+/// Remove an instance by ID
+pub fn remove_instance(id: u32) -> Option<FileBoxInstance> {
+    match INSTANCES.lock() {
+        Ok(mut map) => map.remove(&id),
+        Err(_) => None,
+    }
+}
+
+/// Get mutable access to an instance
+pub fn with_instance_mut<F, R>(id: u32, f: F) -> Result<R, &'static str>
+where
+    F: FnOnce(&mut FileBoxInstance) -> R,
+{
+    match INSTANCES.lock() {
+        Ok(mut map) => match map.get_mut(&id) {
+            Some(instance) => Ok(f(instance)),
+            None => Err("Instance not found"),
+        },
+        Err(_) => Err("Failed to lock instances map"),
+    }
+}
+
+/// Get access to an instance
+pub fn with_instance<F, R>(id: u32, f: F) -> Result<R, &'static str>
+where
+    F: FnOnce(&FileBoxInstance) -> R,
+{
+    match INSTANCES.lock() {
+        Ok(map) => match map.get(&id) {
+            Some(instance) => Ok(f(instance)),
+            None => Err("Instance not found"),
+        },
+        Err(_) => Err("Failed to lock instances map"),
+    }
+}