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/runtime/mod.rs

@@ -5,6 +5,8 @@
 pub mod plugin_config;
 pub mod box_registry;
 pub mod plugin_loader_v2;
+pub mod plugin_loader_unified;
+pub mod plugin_ffi_common;
 pub mod leak_tracker;
 pub mod unified_registry;
 pub mod nyash_runtime;
@@ -17,6 +19,7 @@ mod tests;
 pub use plugin_config::PluginConfig;
 pub use box_registry::{BoxFactoryRegistry, BoxProvider, get_global_registry};
 pub use plugin_loader_v2::{PluginLoaderV2, get_global_loader_v2, init_global_loader_v2};
+pub use plugin_loader_unified::{PluginHost, get_global_plugin_host, init_global_plugin_host, PluginLibraryHandle, PluginBoxType, MethodHandle};
 pub use unified_registry::{get_global_unified_registry, init_global_unified_registry, register_user_defined_factory};
 pub use nyash_runtime::{NyashRuntime, NyashRuntimeBuilder};
 // pub use plugin_box::PluginBox;  // legacy