fix(client): fix MessagePack int_64 encoder dead code branch (#516)

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>

client/addons/messagepack/messagepack.gd

@@ -87,7 +87,7 @@ static func _encode_message(buffer: StreamPeerBuffer, value):
 			elif 0 <= value and value <= (1 << 32) - 1:
 				buffer.put_u8(types["uint_32"])
 				buffer.put_u32(value)
-			elif - (1 << 63) <= value and value < (1 << 63):
+			elif value < 0:
 				buffer.put_u8(types["int_64"])
 				buffer.put_64(value)
 			else: