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/examples/README.md

@@ -0,0 +1,14 @@
+# Examples: Plugin BoxRef Return (v2.2)
+
+- File: `plugin_boxref_return.nyash`
+- Purpose: Demonstrates a plugin method returning a Box (BoxRef/Handle), and passing Box as an argument.
+
+How to run (after full build):
+- Ensure `nyash.toml` includes FileBox with methods:
+  - `copyFrom = { method_id = 7, args = [ { kind = "box", category = "plugin" } ] }`
+  - `cloneSelf = { method_id = 8 }`
+- Build the plugin: `cd plugins/nyash-filebox-plugin && cargo build --release`
+- Run the example: `./target/release/nyash docs/examples/plugin_boxref_return.nyash`
+
+Expected behavior:
+- Creates two FileBox instances (`f`, `g`), writes to `f`, copies content to `g` via `copyFrom`, then closes both.