Replace the double-based fallback in generated integer slots with a
string-to-int64 parser that handles decimal points and exponents
without losing precision near the int64 boundaries.
The old path used std::from_chars<double> and compared against
±9223372036854775808.0, which rounds the int64 max and min so that
valid values are rejected and out-of-range negatives are accepted.
The new helper:
- Parses sign, integer part, optional fraction, and optional exponent.
- Strips trailing zeros to cancel fractional places.
- Rejects non-integral values and overflow using exact uint64_t
arithmetic.
Adds regression tests covering root integer and object-field integer
boundary values, including the cases from issue #19.
weaselbot