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>

docs/reference/boxes-system/p2pbox.md

@@ -0,0 +1,68 @@
+# P2PBox Reference (Phase 9.79 Minimal)
+
+Status: Experimental (Loopback-ready)
+Updated: 2025-08-26
+
+## Overview
+P2PBox is a structured P2P node built on IntentBox + MessageBus + Transport.
+This document covers the minimal API implemented for Phase 9.79.
+
+## Core API
+- new(nodeId: String, transport: String)
+- getNodeId() -> String
+- getTransportType() -> String
+- isReachable(nodeId: String) -> Bool
+- send(to: String, intent: IntentBox) -> ResultBox<Bool, Error>
+- on(intentName: String, handler: MethodBox) -> ResultBox<Bool, Error>
+- getLastIntentName() -> String  // testing helper
+- getLastFrom() -> String        // testing helper
+
+Notes:
+- send() returns ResultBox; Ok(true) on success, Err(message) on failure
+- on() currently wires MethodBox to transport and will call MethodBox.invoke(intent, from)
+- MethodBox.invoke is a stub until Interpreter hook is added (see Roadmap)
+
+## Transport
+- InProcessTransport only (in-proc MessageBus)
+- Transport trait exposes register_intent_handler(intent, cb) used by on()
+
+## Lifecycle
+Typical pattern where handler captures 'me':
+
+```nyash
+static box Node {
+  init { p2p, handle }
+
+  birth() {
+    me.p2p = new P2PBox("alice", "inprocess")
+    me.handle = new MethodBox(me, "onPing")
+    me.p2p.on("ping", me.handle)
+  }
+
+  onPing(intent, from) {
+    // TODO: requires MethodBox.invoke integration with interpreter
+    // print("got " + intent.getName() + " from " + from)
+  }
+}
+```
+
+Destruction order (no strong-cycle):
+- Node (me) drops → fields drop (p2p, handle)
+- P2PBox drops → InProcessTransport Drop → MessageBus unregister
+- Registered handlers released → MethodBox released → me released
+
+## Quick Smoke
+```bash
+cargo build -j32
+./target/debug/nyash local_tests/p2p_self_ping.nyash
+# Expect:
+# last intent: ping
+# last from: alice
+```
+
+## Roadmap
+- MethodBox.invoke → Interpreter hook (global invoker or dedicated API)
+- P2PBox.off(intent) and onOnce(intent, handler)
+- WebSocket/WebRTC transports implementing register_intent_handler
+- Remove testing helpers (getLast*) once full handler path is live
+