feat: Add returns_result support and when pattern-matching design

- Add returns_result.md documenting Result正規化 (returns_result = true)
- Add when-pattern-matching.md with future pattern matching syntax design
- Update E2E tests to use get_value() for HTTP responses
- Update plugin system README with B案 (Result-based) support
- Remove singleton from HttpServerBox and SocketServerBox for stability

This prepares for gradual migration to Result-based error handling,
starting with Net plugin methods, as agreed with ChatGPT5's design.

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

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

docs/reference/plugin-system/README.md

@@ -28,6 +28,9 @@
 - **[net-plugin.md](./net-plugin.md)** - Netプラグイン（HTTP/TCP PoC）
   - GET/POST、ヘッダ、Content-Length、環境変数によるログ
 
+- **[returns-result.md](./returns-result.md)** - 可選のResultBox正規化
+  - `returns_result = true` で成功/失敗を `Ok/Err` に統一（段階導入推奨）
+
 ### ⚙️ 戻り値のResult化（B案サポート）
 - `nyash.toml` のメソッド定義に `returns_result = true` を付けると、
   - 成功: `Ok(value)` の `ResultBox` に包んで返す