The int_64 branch condition `-(1 << 63) <= value` overflowed in
GDScript's signed 64-bit arithmetic, making the branch unreachable.
Negative values beyond int_32 range were incorrectly encoded as
uint_64 (0xcf) instead of int_64 (0xd3). Replaced with `value < 0`.
Updated boundary tests BV-N15 and BV-N16 to expect correct int_64
header byte.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>