Fix thread-safety crashes in city generation under parallel worldgen

[Arilas]

Problem

Each dimension caches a single LostCityTerrainFeature, which owns a single shared ChunkDriver, and IDimensionInfo.setWorld() swaps the shared region reference right before every generation. The feature step of chunk generation can run concurrently for different chunks on worker threads, so there is a race: one thread's driver teardown (the setPrimer(oldRegion, oldChunk) restore at the end of generate()) or region swap can land in the middle of another thread's generation.

Crashes we captured during world creation ("Preparing spawn area"):

• java.lang.NullPointerException: Cannot invoke "ChunkAccess.getPos()" because "this.primer" is null at ChunkDriver.current (via Spheres.fillSphere ← LostCitySphereFeature.place)
• NullPointerException: "this.region" is null at ChunkDriver.getBlock
• IllegalStateException: Requested chunk unavailable during world generation at WorldGenRegion.getChunk via getBlockSafe → updateAdjacent → correct → block
• generation running against another thread's WorldGenRegion (visible as reads/writes 3–8 chunks outside the current region)

This reproduces near-instantly with parallel-worldgen mods (C2ME on the Fabric side is how we hit it), but the race itself exists under vanilla parallel chunk generation too.

Fix

• LostCityFeature.place and LostCitySphereFeature.place now synchronize the setWorld + generate sequence on the shared per-dimension terrain feature instance (both features use the same monitor). Only Lost Cities' own generation is serialized per dimension; the rest of worldgen stays parallel.
• ChunkDriver.getBlockSafe/updateAdjacent treat neighbouring chunks that aren't available in the current WorldGenRegion as air / skip the update instead of throwing — these reads only drive cosmetic connection states (fences, walls, stairs), so a neighbour read can never kill chunk generation anymore.

Verification

• ./gradlew compileJava passes on this branch (1.21.11_neo).
• The identical fix, ported to a Fabric/26.2 build of this codebase, was smoke-tested with dedicated-server world creation on fresh worlds using both the default and biosphere profiles: no exceptions, cities and sphere shells generate. I have not run the NeoForge build in-game.

🤖 Generated with Claude Code

[McJty]

I'm actually working on a big reword of Lost Cities in 1.20.1 branch which I will later port over to 1.21. These changes include work around thread-safety and such so I would wait a bit with this.