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fix(json): preserve the sign of -0 in the integer fast path

Fixes #4053.

parse("-0") returned +0.0 while parse("-0.0") / parse("-0e0") returned
-0.0: the safe-integer fast path in lex_number_end negated the mantissa
as an Int64 (where -0 == 0) before converting to Double. Negate after
the conversion so every spelling of negative zero keeps the IEEE-754
sign bit, per RFC 8259 number semantics. The conversion-then-negation
is exact for the whole safe-integer range.

Deterministic regression tests in lex_number_test.mbt cover every
spelling of negative zero, including values that underflow to zero
(which already preserved the sign).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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