Fix MethodRpc GCHandle leak

[naxturnal]

Problem

GCHandle.Alloc(this) pins the MethodRpc instance forever, causing a memory leak over time.

Solution

Replaced with a static Dictionary<uint, MethodRpc> cache. Each RPC gets a unique sender ID stored as an instance field. Added cleanup methods:

• Unregister() - removes individual RPC from cache
• UnregisterPlugin(plugin) - cleans up all RPCs for a plugin on unload
• CleanupDeadReferences() - full cache clear

Also added a debug assertion to catch sender ID collisions.

Benefits

• Fixes the memory leak
• Simpler IL generation