Add extended lobbies, relay hardening, and public registry support

[mrfox-1]

Summary

Evolves the existing GE02/v2 matchmaker into the relay used by the separate GoldenEye Recomp
Online launcher while preserving opaque System Link gameplay packets and legacy clients.

• reports live room membership instead of stale host-advertised counts;
• adds backward-compatible extended lobby metadata, private invite-code queries, build IDs, and
  server capability/version reporting;
• rejects malformed/unregistered relay traffic and rate-limits public queries and packet bursts;
• adds a documented Windows/Linux CLI plus non-root Docker and Compose deployment;
• sends optional lightweight relay heartbeats to the separate public directory service;
• advertises registry support so a managed launcher can reject an incompatible server.

Protocol types 1-8 and their gameplay behavior are unchanged. Types 9-11 are additive lobby
registration/query messages; type 12 is used only from the relay to the public registry.

Verification

• Windows build and three server tests pass locally and in the coordinated launcher CI.
• Real relay integration covers public/private visibility, wrong invite codes, live player counts,
  malformed traffic, and rate limiting.
• The launcher-managed registration test starts this executable, receives its real UDP heartbeat,
  reads the HTTP directory, advertises a lobby, and discovers that lobby.
• Linux non-root container build and entry point pass in GitHub Actions.

Coordinated launcher: https://github.com/mrfox-1/GoldenEye-Recomp-Online