JSON parser DoS prevention

As reported by Claude review, our current JSON parser does not prevent large exponents from exhausting our memory.
The solution proposed by Claude, however, is inflexible: cap the exponent.
For most regular uses that would be OK. For schema validation, though, that might be too restrictive.
Instead, I suggest we just keep the exponent as a separate field for JSON numbers.
The trade off:

• We get arbitrarily large numbers with efficient-ish representation
• We allow users to define schemas with arbitrary numbers

• We need to implement basic operations and might need some float-like behavior in places

この issue ではファイル、テスト、エントリポイントが指定されていません。まず JSON パーサーとその数値表現を見つけ、次に指数を保持する範囲と必要な基本操作を特定します。大きな指数によってメモリ枯渇が発生しなくなり、スキーマ検証が引き続き任意の数値を受け入れれば完了です。