refactor: 大型プラグインのモジュール分割によるコード品質向上

nyash-json-plugin:
- 796行の単一ファイルから6モジュール構造へ分割
- constants.rs, provider.rs, doc_box.rs, node_box.rs, tlv_helpers.rs, ffi.rsに責任分離
- 最大ファイルサイズを374行に削減（53%削減）
- 共有状態管理をprovider.rsに集約

nyash-net-plugin:
- 1112行の巨大ファイルから17ファイル構造へ分割
- boxesサブディレクトリでBox実装を整理（server, client, request, response, socket系）
- 最大ファイルサイズを290行に削減（74%削減）
- logging, tlv, http_helpers等の共通機能を独立モジュール化

両プラグインともビルド成功確認済み、完全な後方互換性を維持

🤖 Generated with Claude Code
Co-Authored-By: Claude <noreply@anthropic.com>

plugins/nyash-json-plugin/src/tlv_helpers.rs

@@ -0,0 +1,129 @@
+//! TLV (Type-Length-Value) serialization helpers
+
+use crate::constants::*;
+
+pub fn write_tlv_result(payloads: &[(u8, &[u8])], result: *mut u8, result_len: *mut usize) -> i32 {
+    if result_len.is_null() {
+        return E_ARGS;
+    }
+    let mut buf: Vec<u8> =
+        Vec::with_capacity(4 + payloads.iter().map(|(_, p)| 4 + p.len()).sum::<usize>());
+    buf.extend_from_slice(&1u16.to_le_bytes());
+    buf.extend_from_slice(&(payloads.len() as u16).to_le_bytes());
+    for (tag, payload) in payloads {
+        buf.push(*tag);
+        buf.push(0);
+        buf.extend_from_slice(&(payload.len() as u16).to_le_bytes());
+        buf.extend_from_slice(payload);
+    }
+    unsafe {
+        let needed = buf.len();
+        if result.is_null() || *result_len < needed {
+            *result_len = needed;
+            return E_SHORT;
+        }
+        std::ptr::copy_nonoverlapping(buf.as_ptr(), result, needed);
+        *result_len = needed;
+    }
+    OK
+}
+
+pub fn write_u32(v: u32, result: *mut u8, result_len: *mut usize) -> i32 {
+    if result_len.is_null() {
+        return E_ARGS;
+    }
+    unsafe {
+        if result.is_null() || *result_len < 4 {
+            *result_len = 4;
+            return E_SHORT;
+        }
+        let b = v.to_le_bytes();
+        std::ptr::copy_nonoverlapping(b.as_ptr(), result, 4);
+        *result_len = 4;
+    }
+    OK
+}
+
+pub fn write_tlv_void(result: *mut u8, result_len: *mut usize) -> i32 {
+    // Align with common helpers: use tag=9 for void/host-handle-like empty
+    write_tlv_result(&[(9u8, &[])], result, result_len)
+}
+
+pub fn write_tlv_i64(v: i64, result: *mut u8, result_len: *mut usize) -> i32 {
+    write_tlv_result(&[(3u8, &v.to_le_bytes())], result, result_len)
+}
+
+pub fn write_tlv_bool(v: bool, result: *mut u8, result_len: *mut usize) -> i32 {
+    write_tlv_result(&[(1u8, &[if v { 1u8 } else { 0u8 }])], result, result_len)
+}
+
+pub fn write_tlv_handle(
+    type_id: u32,
+    instance_id: u32,
+    result: *mut u8,
+    result_len: *mut usize,
+) -> i32 {
+    let mut payload = Vec::with_capacity(8);
+    payload.extend_from_slice(&type_id.to_le_bytes());
+    payload.extend_from_slice(&instance_id.to_le_bytes());
+    write_tlv_result(&[(8u8, &payload)], result, result_len)
+}
+
+pub fn write_tlv_string(s: &str, result: *mut u8, result_len: *mut usize) -> i32 {
+    write_tlv_result(&[(6u8, s.as_bytes())], result, result_len)
+}
+
+pub fn read_arg_string(args: *const u8, args_len: usize, n: usize) -> Option<String> {
+    if args.is_null() || args_len < 4 {
+        return None;
+    }
+    let buf = unsafe { std::slice::from_raw_parts(args, args_len) };
+    let mut off = 4usize;
+    for i in 0..=n {
+        if buf.len() < off + 4 {
+            return None;
+        }
+        let tag = buf[off];
+        let size = u16::from_le_bytes([buf[off + 2], buf[off + 3]]) as usize;
+        if buf.len() < off + 4 + size {
+            return None;
+        }
+        if i == n {
+            if tag != 6 {
+                return None;
+            }
+            let s = String::from_utf8_lossy(&buf[off + 4..off + 4 + size]).to_string();
+            return Some(s);
+        }
+        off += 4 + size;
+    }
+    None
+}
+
+pub fn read_arg_i64(args: *const u8, args_len: usize, n: usize) -> Option<i64> {
+    if args.is_null() || args_len < 4 {
+        return None;
+    }
+    let buf = unsafe { std::slice::from_raw_parts(args, args_len) };
+    let mut off = 4usize;
+    for i in 0..=n {
+        if buf.len() < off + 4 {
+            return None;
+        }
+        let tag = buf[off];
+        let size = u16::from_le_bytes([buf[off + 2], buf[off + 3]]) as usize;
+        if buf.len() < off + 4 + size {
+            return None;
+        }
+        if i == n {
+            if tag != 3 || size != 8 {
+                return None;
+            }
+            let mut b = [0u8; 8];
+            b.copy_from_slice(&buf[off + 4..off + 12]);
+            return Some(i64::from_le_bytes(b));
+        }
+        off += 4 + size;
+    }
+    None
+}