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/development/current/PLUGIN_LOADER_MIGRATION_PLAN.md

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+# Plugin Loader Migration Plan
+
+## Overview
+Consolidate three plugin loader implementations into a single unified system.
+
+## Current State
+1. **plugin_loader_v2.rs** (906 lines) - Main BID-FFI plugin system
+2. **plugin_loader.rs** (1217 lines) - Builtin box dynamic loading
+3. **plugin_loader_legacy.rs** (299 lines) - Legacy host vtable system
+
+Total: ~2400 lines of code with significant duplication
+
+## Target State
+- **plugin_loader_unified.rs** - Single unified loader (~800 lines)
+- **plugin_ffi_common.rs** - Shared FFI utilities (~300 lines)
+- Total: ~1100 lines (55% reduction)
+
+## Migration Steps
+
+### Phase 1: Infrastructure (DONE)
+- [x] Create plugin_loader_unified.rs skeleton
+- [x] Create plugin_ffi_common.rs for shared types
+- [x] Update runtime/mod.rs
+- [x] Basic compilation check
+
+### Phase 2: Common Pattern Extraction
+- [ ] Extract TLV encoding/decoding to plugin_ffi_common
+- [ ] Extract handle management patterns
+- [ ] Extract memory management utilities
+- [ ] Extract error handling patterns
+
+### Phase 3: BID Plugin Migration
+- [ ] Port PluginBoxV2 implementation
+- [ ] Port BID FFI invoke mechanism
+- [ ] Port plugin loading logic
+- [ ] Port configuration parsing
+- [ ] Migrate tests
+
+### Phase 4: Builtin Plugin Migration
+- [ ] Port FileBoxProxy and related proxies
+- [ ] Port dynamic library loading for builtins
+- [ ] Port builtin-specific FFI patterns
+- [ ] Migrate feature flags handling
+
+### Phase 5: Legacy Plugin Migration
+- [ ] Port host vtable implementation
+- [ ] Port legacy box creation
+- [ ] Decide on deprecation timeline
+
+### Phase 6: Integration
+- [ ] Update all references to old loaders
+- [ ] Ensure backward compatibility
+- [ ] Performance benchmarking
+- [ ] Documentation update
+
+### Phase 7: Cleanup
+- [ ] Remove old plugin loaders
+- [ ] Remove redundant tests
+- [ ] Update CLAUDE.md
+- [ ] Final code review
+
+## Technical Decisions
+
+### Unified Plugin Type Detection
+```rust
+fn detect_plugin_type(path: &str) -> PluginType {
+    // 1. Check file extension (.bid, .legacy, .builtin)
+    // 2. Check nyash-box.toml for type field
+    // 3. Probe for known symbols
+    // 4. Default to BID
+}
+```
+
+### Handle Management Strategy
+- Use generic `PluginHandle<T>` for all plugin types
+- Centralized handle registry in plugin_ffi_common
+- Reference counting with Arc<Mutex<T>>
+
+### Error Handling
+- Convert all errors to Result<T, String> at boundaries
+- Plugin-specific errors wrapped with context
+- Consistent error messages across all plugin types
+
+## Risk Mitigation
+
+### Backward Compatibility
+- Keep old loaders under feature flags during migration
+- Provide compatibility shims if needed
+- Extensive testing with existing plugins
+
+### Performance
+- Benchmark before and after migration
+- Profile hot paths (box creation, method dispatch)
+- Optimize only after correctness verified
+
+### Testing Strategy
+1. Unit tests for each component
+2. Integration tests with real plugins
+3. E2E tests with full applications
+4. Regression tests for edge cases
+
+## Success Metrics
+- [ ] Code reduction: 50%+ fewer lines
+- [ ] All existing tests pass
+- [ ] No performance regression
+- [ ] Cleaner API surface
+- [ ] Better error messages
+- [ ] Easier to add new plugin types
+
+## Timeline
+- Phase 1-2: Day 1 (Today)
+- Phase 3-4: Day 2-3
+- Phase 5-6: Day 4
+- Phase 7: Day 5
+
+Total: 5 days for complete migration