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>