Fix computeChatChecksum returning unsigned byte for i8 schema (closes #1482)

[mio-rei]

Summary

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 checksum exceeded 127, the protocol writer rejected it with RangeError: The value of \"value\" is out of range. It must be >= -128 and <= 127. Received N, breaking any signed slash command on MC 1.21.11.

Closes #1482.

Fix

Convert the unsigned byte to signed range before returning:

const unsigned = checksum & 0xff
const signed = unsigned > 127 ? unsigned - 256 : unsigned
return signed === 0 ? 1 : signed

Test plan

• Locally patched node_modules/minecraft-protocol/src/datatypes/checksums.js with the same conversion against MC 1.21.11 vanilla server (LAN), bot has been sending /say / /tell for ~14 days without RangeError.
• Adding a unit test to the repo would be nice — happy to follow up if reviewers want one in this PR.

Notes

Issue #1482 has the full repro steps (versions, error, root cause). This PR is the minimal fix; the value-zero guard (return 1) is preserved.