ft: parallel entity processing optimizations and stability fixes

[Xytronix]

Summary

Adds parallel entity processing optimizations and stability fixes for high-entity-count servers.

Type of change

• Bug fix (non-breaking)
• New feature (non-breaking)
• Breaking change (requires documentation/migration)
• Chore / refactor
• Documentation only

Implementation details

• MixinTickingThread (isInThread() relaxation): Overrides isInThread() to return true for ForkJoinWorkerThreads belonging to ForkJoinPool.commonPool(). Without this, parallel entity ticking deadlocks: the world thread blocks waiting for pool tasks, while pool tasks try to bounce World method calls (chunk lookups, player queries) back to the blocked world thread via supplyAsync(..., this).join().
• MixinSpatialSystem: Replaces the sequential forEachChunk position collection in SpatialSystem.tick() with parallel per-chunk collection into thread-local buffers, followed by sequential merge into SpatialData. It falls back to sequential for ≤2 chunks.
• ParallelSpatialCollector: Utility that submits per-chunk collection tasks to ForkJoinPool.commonPool(), collects (position, ref) pairs into local lists, then merges into SpatialData sequentially.
• MixinSteeringSystem: Enables isParallel() for SteeringSystem. It is gated behind a threshold (default 64) and is safe production, but can cause a crash if debugging enabled (non-atomic save).
• MixinEntityViewer: Silently drops queueUpdate/queueRemove calls for entities not in the visibility set, instead of throwing IllegalArgumentException (which disconnects the player).
• WatchDog optimisation: prevented crash by removing Universe.get().getDefaultWorld() from constructor if universe is not ready, adds null guards on watchdogThread.interrupt() in unregisterService(), lastDefaultWorld.getName() in start() and fixes integer division for timeout logging.

Checklist

• Code compiles successfully
• Pull request does not contain unrelated changes
• Changes follow this project's code style (use spotlessApply when necessary)
• Changes are tested or will be tested after submission