Problem

When players join a Folia or CanvasMC server, a ConcurrentModificationException is thrown in CacheableRegistry.getId():

java.util.ConcurrentModificationException
    at java.base/java.util.HashMap.computeIfAbsent(HashMap.java:1230)
    at MapModCompanion.jar//...spigot.IdRegistry$CacheableRegistry.getId(IdRegistry.java:26)
    at MapModCompanion.jar//...spigot.XaeroHandler.sendPacket(XaeroHandler.java:67)
    at MapModCompanion.jar//...spigot.XaeroHandler.onPlayerJoined(XaeroHandler.java:56)

Folia (and forks like CanvasMC) uses regionized multithreading, meaning PlayerJoinEvent can be fired concurrently from different region tick threads. The CacheableRegistry cache was backed by a plain HashMap, which is not thread-safe. Concurrent calls to HashMap.computeIfAbsent() from multiple threads cause a ConcurrentModificationException.

Full log: https://mclo.gs/bYg8IkD

Fix

Replace HashMap with ConcurrentHashMap in CacheableRegistry:

- private final Map<String, Integer> cache = new HashMap<>();
+ private final Map<String, Integer> cache = new ConcurrentHashMap<>();

ConcurrentHashMap.computeIfAbsent() is atomic and thread-safe, which prevents the concurrent modification issue on multi-threaded server implementations.

Impact

• Fixes exception on player join for Folia, CanvasMC, and other regionized server forks.
• No behavioral change on standard Spigot/Paper servers (single-threaded tick loop).