Fix async mob spawning deadlock on structure_starts chunk loads

[XUANHLGG]

I ran into a server hang where multiple Async-Tick-Pool-Threads were stuck on Sync load chunk to status minecraft:structure_starts in world minecraft:overworld, and the server was killed by watchdog after about one minute.

The original implementation had two problems:
tickChunksSpawn() could run natural mob spawning on the async pool
non-main-thread async$getChunk() could still reach create=true or high-risk chunk states such as structure_starts, which could make async workers wait for main-thread chunk loading
That creates a wait cycle: async worker threads wait for chunk loading, while the server thread can end up waiting for async worker completion.

This PR changes that behavior by:
adding enableAsyncMobSpawning as a separate config option
defaulting async natural mob spawning to disabled when C2ME is detected via getDefaultAsyncMobSpawning()
blocking unsafe async chunk requests in async$getChunk()
making shortcutGetChunkNow() only read already-loaded FULL chunks instead of triggering generation from worker threads
forcing natural mob spawning to fall back to synchronous execution when async spawning becomes unsafe in tickChunksSpawn()