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);

src/interpreter/methods/io_methods.rs

@@ -10,6 +10,7 @@
 use super::super::*;
 use crate::box_trait::{ResultBox, StringBox, NyashBox};
 use crate::boxes::FileBox;
+use crate::bid::plugin_box::PluginFileBox;
 
 impl NyashInterpreter {
     /// FileBoxのメソッド呼び出しを実行
@@ -105,4 +106,73 @@ impl NyashInterpreter {
             })
         }
     }
+
+    /// PluginFileBoxのメソッド呼び出しを実行 (BID-FFI system)
+    /// Handles plugin-backed file I/O operations via FFI interface
+    pub(in crate::interpreter) fn execute_plugin_file_method(&mut self, plugin_file_box: &PluginFileBox, method: &str, arguments: &[ASTNode]) 
+        -> Result<Box<dyn NyashBox>, RuntimeError> {
+        match method {
+            "read" => {
+                if !arguments.is_empty() {
+                    return Err(RuntimeError::InvalidOperation {
+                        message: format!("read() expects 0 arguments, got {}", arguments.len()),
+                    });
+                }
+                // Read the entire file content
+                match plugin_file_box.read_bytes(8192) { // Read up to 8KB
+                    Ok(bytes) => {
+                        let content = String::from_utf8_lossy(&bytes).to_string();
+                        Ok(Box::new(StringBox::new(content)))
+                    }
+                    Err(e) => Err(RuntimeError::InvalidOperation {
+                        message: format!("Plugin read failed: {:?}", e),
+                    })
+                }
+            }
+            "write" => {
+                if arguments.len() != 1 {
+                    return Err(RuntimeError::InvalidOperation {
+                        message: format!("write() expects 1 argument, got {}", arguments.len()),
+                    });
+                }
+                let content = self.execute_expression(&arguments[0])?;
+                let text = content.to_string_box().value;
+                match plugin_file_box.write_bytes(text.as_bytes()) {
+                    Ok(bytes_written) => Ok(Box::new(StringBox::new("OK".to_string()))),
+                    Err(e) => Err(RuntimeError::InvalidOperation {
+                        message: format!("Plugin write failed: {:?}", e),
+                    })
+                }
+            }
+            "exists" => {
+                if !arguments.is_empty() {
+                    return Err(RuntimeError::InvalidOperation {
+                        message: format!("exists() expects 0 arguments, got {}", arguments.len()),
+                    });
+                }
+                // Plugin FileBox doesn't have exists() method in current implementation
+                // Return true if we can read (approximate)
+                match plugin_file_box.read_bytes(1) {
+                    Ok(_) => Ok(Box::new(crate::box_trait::BoolBox::new(true))),
+                    Err(_) => Ok(Box::new(crate::box_trait::BoolBox::new(false))),
+                }
+            }
+            "close" => {
+                if !arguments.is_empty() {
+                    return Err(RuntimeError::InvalidOperation {
+                        message: format!("close() expects 0 arguments, got {}", arguments.len()),
+                    });
+                }
+                match plugin_file_box.close() {
+                    Ok(()) => Ok(Box::new(StringBox::new("OK".to_string()))),
+                    Err(e) => Err(RuntimeError::InvalidOperation {
+                        message: format!("Plugin close failed: {:?}", e),
+                    })
+                }
+            }
+            _ => Err(RuntimeError::InvalidOperation {
+                message: format!("Unknown method '{}' for PluginFileBox", method),
+            })
+        }
+    }
 }