feat(phase-9.75g-0): Complete BID-FFI Day 5 - Plugin method calling system

## 🎉 Major Achievement
- BID-FFI FileBox plugin fully functional with Nyash integration
- Complete plugin-backed file I/O operations working
- Successful write/read operations via FFI interface

## ✅ What Works
- Plugin loading from nyash.toml configuration
- FileBox plugin instantiation: `new FileBox(path)`
- Method calls: `f.write("text")`, `f.read()`
- Complete round-trip: Nyash → Plugin → File → Plugin → Nyash

## 🔧 Implementation Details
- Added PluginFileBox method dispatch in execute_method_call()
- Implemented execute_plugin_file_method() for read/write/exists/close
- Fixed "Cannot call method on non-instance type" error
- Plugin methods work via TLV encoding/FFI/decoding

## 🚨 Known Issue (Next Phase)
Current implementation uses hardcoded method names (read/write/exists/close).
This violates BID-FFI dynamic principles - methods should be discovered
from plugin metadata, not hardcoded in Nyash interpreter.

## 📊 Test Results
```
local f
f = new FileBox("test.txt")
f.write("Hello from Nyash via plugin\!")
print("READ=" + f.read())
# Output: READ=Hello from Nyash via plugin\!
```

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

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

src/interpreter/expressions/calls.rs

@@ -255,6 +255,11 @@ impl NyashInterpreter {
             return self.execute_file_method(file_box, method, arguments);
         }
         
+        // PluginFileBox method calls (BID-FFI system)
+        if let Some(plugin_file_box) = obj_value.as_any().downcast_ref::<crate::bid::plugin_box::PluginFileBox>() {
+            return self.execute_plugin_file_method(plugin_file_box, method, arguments);
+        }
+        
         // ResultBox method calls
         if let Some(result_box) = obj_value.as_any().downcast_ref::<crate::box_trait::ResultBox>() {
             return self.execute_result_method(result_box, method, arguments);