perf/safety: convert parseStr from recursive to iterative to prevent stack overflow

Problem

parseStr in Lck/Json/Tokenizer.lean is written as a recursive CPS function. While the continuation wrapping is proven zero-cost at runtime (proofs are erased), the recursion itself is structural on the input — meaning for a string literal of length n, the Lean runtime creates n stack frames.

For pathologically large string values this can overflow the stack.

Expected fix

Rewrite parseStr as an iterative (tail-recursive or explicit loop) accumulator, maintaining the same CPS interface for callers. The proof structure can be adapted to use an inductive invariant on the loop state.

References

• Flagged by AI code review on PR #8

Comienza en Lck/Json/Tokenizer.lean, en la implementación CPS recursiva de parseStr, y luego inspecciona cómo dependen los llamadores de su interfaz CPS. Reescribe el análisis de cadenas como un acumulador iterativo conservando esa interfaz, adapta la demostración en torno a un invariante inductivo del bucle y verifica que los literales de cadena grandes ya no corren el riesgo de provocar un desbordamiento de pila.