feat(plugin): Fix plugin BoxRef return and Box argument support

- Fixed deadlock in FileBox plugin copyFrom implementation (single lock)
- Added TLV Handle (tag=8) parsing in calls.rs for returned BoxRefs
- Improved plugin loader with config path consistency and detailed logging
- Fixed loader routing for proper Handle type_id/fini_method_id resolution
- Added detailed logging for TLV encoding/decoding in plugin_loader_v2

Test docs/examples/plugin_boxref_return.nyash now works correctly:
- cloneSelf() returns FileBox Handle properly
- copyFrom(Box) accepts plugin Box arguments
- Both FileBox instances close and fini correctly

🤖 Generated with [Claude Code](https://claude.ai/code)

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

docs/reference/architecture/plugin-migration-guide-v2-summary.md

@@ -0,0 +1,45 @@
+# Plugin Migration Guide v2 Enhancement Summary
+
+## What was accomplished
+
+This task involved enhancing the existing `docs/plugin-migration-request.md` with comprehensive implementation guidance based on the issue requirements. 
+
+## Key improvements made:
+
+### 1. **Comprehensive nyash.toml explanation**
+- Added detailed explanation of the `from`/`to` type conversion system
+- Explained TLV (Type-Length-Value) encoding with specific tag mappings
+- Provided clear examples using FileBox as reference
+
+### 2. **Detailed implementation examples**
+- Added complete Rust code examples for TLV parsing
+- Showed real HttpClientBox plugin implementation patterns
+- Included proper error handling and memory management examples
+
+### 3. **Structured migration priorities**
+- **Phase 1**: Network boxes (HttpClientBox, SocketBox) - highest priority
+- **Phase 2**: GUI boxes (EguiBox, Canvas) - platform dependent  
+- **Phase 3**: Special purpose boxes (TimerBox, QRBox) - independent
+
+### 4. **Testing and validation guidelines**
+- Complete testing workflow with plugin-tester
+- Real Nyash code examples for validation
+- Troubleshooting guidance for common mistakes
+
+### 5. **Reference implementation guidance**
+- FileBox plugin as the gold standard example
+- Specific file paths for all reference materials
+- Success tips and common pitfalls
+
+## Document statistics:
+- **Length**: 368 lines (vs ~200 lines originally)
+- **Code examples**: 16 code blocks with real implementation patterns
+- **Comprehensive coverage**: TLV, nyash.toml, FFI, testing, and reference materials
+
+## Validation:
+- All key sections verified to be present
+- Code examples cover the full implementation pipeline
+- References to FileBox plugin success story maintained
+- HttpClientBox positioned as Phase 1 priority target
+
+The guide now serves as a complete reference for migrating any builtin Box to a plugin, with FileBox as the proven template and HttpClientBox as the next target.