security: deep nesting causes stack overflow in JSON parser

Problem

The JSON parser (parseValue/parseArrayElems/parseObjectElems) is mutually recursive with no depth limit. Deeply nested JSON (e.g., [[[[...10000 levels...]]]]) will exhaust the Lean runtime stack.

This is distinct from the DoS risk in #13 (which is about exponent arithmetic) — this is about recursion depth on the parse stack.

Expected fix

Add a configurable maximum nesting depth to the parser (defaulting to a safe value like 512 or 1024). Return a ParseError.nestingDepthExceeded on overflow.

References

• Related to: #13 (DoS prevention)
• Flagged by AI code review on PR #8

まず JSON パーサー関数 parseValue、parseArrayElems、parseObjectElems を読み、それらの相互再帰と既存の ParseError 処理を追跡します。設定可能な深さ制限をどのように渡すか、また適切なデフォルト値は何かを定義し、その後、深くネストされた入力がランタイムスタックを枯渇させずに ParseError.nestingDepthExceeded を返すことを確認します。