json: parse quoted integers exactly

[fallintoplace]

Summary

Parse quoted integer strings exactly in the JSON helpers instead of routing decimal/exponent forms through f64.

Why

parse_int_from_str was exact for plain integer strings, but strings like "1e5" or "9007199254740993.0" fell back to f64. That meant values above the 2^53 precision boundary could be silently rounded during JSON decode.

What changed

• replace the f64 fallback with an exact decimal/scientific parser for quoted integer strings
• keep accepting the integer-like string forms the helpers already supported, including zero-fraction decimals and scientific notation
• handle exact negative exponents like "1200e-2"
• add regression coverage for 2^53 boundary cases, u64::MAX with .0, and non-integral exponent strings

Validation

• cargo test -p buffa

[github-actions]

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