Fix race condition in ForgeZetaEventBus (#52)

[MauveAvenger]

Problem

ForgeZetaEventBus.convertedHandlers uses Object2ObjectOpenHashMap which is not thread-safe. During mod loading with many mods (370+), concurrent calls to subscribeMethod() and unsubscribeMethod() can corrupt the map, causing:

java.lang.ArrayIndexOutOfBoundsException: Index 256 out of bounds for length 129
    at it.unimi.dsi.fastutil.objects.Object2ObjectOpenHashMap.rehash(...)
    at org.violetmoon.zetaimplforge.event.ForgeZetaEventBus.subscribeMethod(...)

Relates to #52

Fix

Replace Object2ObjectOpenHashMap with ConcurrentHashMap. This is a minimal change that makes the map operations thread-safe without changing the API or behavior.

Testing

• Tested with 370+ mod modpack that previously failed ~50% of startups
• Server now starts reliably with no race condition crashes
• No performance impact observed (this is startup-time code, not hot path)

Alternatives Considered

• Synchronized blocks around map access - would work but ConcurrentHashMap is cleaner
• Making the whole class synchronized - overkill for this use case

Happy to adjust the approach if you'd prefer something different.