feat: プラグインFFIとTypeBox/TLVテストの拡張

- plugin_ffi_common: デバッグ情報とコメント追加
- typebox_tlv_diff: 差分テストの詳細化

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

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

src/runtime/plugin_ffi_common.rs

@@ -1,6 +1,8 @@
 //! Common FFI helpers for Plugin system
 //! Minimal TLV utilities extracted for unified facade usage.
 
+use crate::box_trait::NyashBox;
+
 /// Encode empty TLV arguments: version=1, argc=0
 pub fn encode_empty_args() -> Vec<u8> { vec![1u8, 0, 0, 0] }
 
@@ -12,6 +14,22 @@ pub fn encode_tlv_header(argc: u16) -> Vec<u8> {
     buf
 }
 
+/// Encode a slice of NyashBox arguments into TLV buffer (v1)
+/// Policy: prefer i64 numeric when coercible; otherwise UTF-8 string; otherwise to_string_box()
+pub fn encode_args(args: &[Box<dyn NyashBox>]) -> Vec<u8> {
+    let mut buf = encode_tlv_header(args.len() as u16);
+    for a in args {
+        if let Some(i) = crate::runtime::semantics::coerce_to_i64(a.as_ref()) {
+            encode::i64(&mut buf, i);
+        } else if let Some(s) = crate::runtime::semantics::coerce_to_string(a.as_ref()) {
+            encode::string(&mut buf, &s);
+        } else {
+            encode::string(&mut buf, &a.to_string_box().value);
+        }
+    }
+    buf
+}
+
 /// Simple helpers for common primitive returns
 pub mod decode {
     /// Try to parse a u32 instance id from an output buffer (little-endian).