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/backend/vm_instructions.rs

@@ -480,10 +480,10 @@ impl VM {
             nyash_args.push(arg_value.to_nyash_box());
         }
         
-        // Route through plugin loader v2 (also handles env.* stubs)
-        let loader = crate::runtime::get_global_loader_v2();
-        let loader = loader.read().map_err(|_| VMError::InvalidInstruction("Plugin loader lock poisoned".into()))?;
-        match loader.extern_call(iface_name, method_name, &nyash_args) {
+        // Route through unified plugin host (delegates to v2, handles env.* stubs)
+        let host = crate::runtime::get_global_plugin_host();
+        let host = host.read().map_err(|_| VMError::InvalidInstruction("Plugin host lock poisoned".into()))?;
+        match host.extern_call(iface_name, method_name, &nyash_args) {
             Ok(Some(result_box)) => {
                 if let Some(dst_id) = dst {
                     self.set_value(dst_id, VMValue::from_nyash_box(result_box));