Fix computeChatChecksum returning unsigned byte for i8 schema (#1485)

The chat_command_signed packet schema defines the checksum as i8 (-128..127),
but computeChatChecksum was returning the result of '& 0xff' (0..255). When
the masked value exceeded 127, sending any signed chat command crashed with
RangeError on MC 1.21.11.

Convert the unsigned byte to signed range before returning. Closes #1482.

Author: mio-rei

src/datatypes/checksums.js

@@ -12,9 +12,11 @@ function computeChatChecksum (lastSeenMessages) {
       checksum = (31 * checksum + sigHash) & 0xffffffff
     }
   }
-  // Convert to byte
-  const result = checksum & 0xff
-  return result === 0 ? 1 : result
+  // Convert to signed byte (i8: -128..127) to match the chat_command_signed packet schema.
+  // The previous `& 0xff` produced 0..255 which causes RangeError when value > 127.
+  const unsigned = checksum & 0xff
+  const signed = unsigned > 127 ? unsigned - 256 : unsigned
+  return signed === 0 ? 1 : signed
 }
 
 module.exports = { computeChatChecksum }