refactor: Major interpreter modularization and P2PBox enhancements

Major Interpreter Refactoring:
- Split core.rs (373 lines removed) into focused modules
- Split expressions/calls.rs (460 lines removed) into cleaner structure
- Added new modules: calls.rs, errors.rs, eval.rs, methods_dispatch.rs, state.rs
- Improved separation of concerns across interpreter components

P2PBox Enhancements:
- Added on_once() for one-time event handlers
- Added off() for handler deregistration
- Implemented handler flags with AtomicBool for thread-safe management
- Added loopback testing cache (last_from, last_intent_name)
- Improved Arc-based state sharing for transport and handlers

Plugin Loader Unification (In Progress):
- Created plugin_loader_unified.rs skeleton
- Created plugin_ffi_common.rs for shared FFI utilities
- Migration plan documented (2400 lines → 1100 lines target)

MIR & VM Improvements:
- Enhanced modularized MIR builder structure
- Added BoxCall dispatch improvements
- Better separation in builder modules

Documentation Updates:
- Added Phase 9.79a unified box dispatch plan
- Created plugin loader migration plan
- Updated CURRENT_TASK.md with latest progress

All tests passing (180 tests) - ready for next phase of refactoring

🤖 Generated with Claude Code

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

src/interpreter/state.rs

@@ -0,0 +1,42 @@
+use crate::instance_v2::InstanceBox;
+use std::collections::{HashMap, HashSet};
+use std::sync::{Arc, Mutex, RwLock};
+use super::{BoxDeclaration, StaticBoxDefinition};
+
+/// スレッド間で共有される状態
+#[derive(Clone)]
+pub struct SharedState {
+    /// 🌍 GlobalBox - すべてのトップレベル関数とグローバル変数を管理
+    pub global_box: Arc<Mutex<InstanceBox>>,
+
+    /// Box宣言のレジストリ（読み込みが多いのでRwLock）
+    pub box_declarations: Arc<RwLock<HashMap<String, BoxDeclaration>>>,
+
+    /// 🔥 静的関数のレジストリ（読み込みが多いのでRwLock）
+    pub static_functions: Arc<RwLock<HashMap<String, HashMap<String, crate::ast::ASTNode>>>>,
+
+    /// 🔥 Static Box定義レジストリ（遅延初期化用）
+    pub static_box_definitions: Arc<RwLock<HashMap<String, StaticBoxDefinition>>>,
+
+    /// 読み込み済みファイル（重複防止）
+    pub included_files: Arc<Mutex<HashSet<String>>>,
+}
+
+impl SharedState {
+    /// 新しい共有状態を作成
+    pub fn new() -> Self {
+        let global_box = InstanceBox::new(
+            "Global".to_string(),
+            vec![],          // フィールド名（空から始める）
+            HashMap::new(),  // メソッド（グローバル関数）
+        );
+
+        Self {
+            global_box: Arc::new(Mutex::new(global_box)),
+            box_declarations: Arc::new(RwLock::new(HashMap::new())),
+            static_functions: Arc::new(RwLock::new(HashMap::new())),
+            static_box_definitions: Arc::new(RwLock::new(HashMap::new())),
+            included_files: Arc::new(Mutex::new(HashSet::new())),
+        }
+    }
+}