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/development/roadmap/phases/phase-5/phase5_2_static_main_lowering.md

@@ -0,0 +1,24 @@
+# Phase 5.2: Lowering for static box Main (BoxDeclaration → main body)
+
+Summary:
+- static box Main { main() { ... } } を MirBuilder で受け、main() の body を Program として lowering する経路を実装します。
+- 目的は `--dump-mir` が static Main 形式のサンプルでも通り、VM 実行にも到達すること。
+
+Scope:
+- AST: BoxDeclaration(is_static=true, name=Main) を検出 → 同名 main() を探して Program 化
+- Lowering: 発見した body を既存の Program lowering に渡す（関数単位でOK）
+- Tests: local_tests/mir_loop_no_local.nyash（static Main）で dump/VM が通る
+
+Tasks:
+- [ ] MirBuilder: static Main → Program lowering 経路
+- [ ] MirPrinter/Verifier: 必要なら修正
+- [ ] サンプル/スナップショットの点検
+
+Acceptance Criteria:
+- `nyash --dump-mir ./local_tests/mir_loop_no_local.nyash` が成功
+- `nyash --backend vm ./local_tests/mir_loop_no_local.nyash` が成功
+
+References:
+- #33, #35
+- docs/guides/how-to-build-native/copilot_issues.txt
+