wip(assembler): round S8 -- region rebuilds no longer stall admission; result backlog capped

voxel.CoreRegionConcurrentAdmission (default on): tiles independent of the
entering region keep submitting while a region page compiles; pages are
immutable shared_ptr owners so running readers survive eviction. 6 m/s: 11
whole-pass skips -> 0, walkable ring ready at every centre change.
voxel.StreamingResultBacklogCap=256 bounds finished results (slow-apply worst
case 125 -> 76 MB). Spawn unchanged at ~7.7 s: LOD8 is 46% of worker time.
At 60 m/s + teleport the collision ring is outrun (102/113 centres).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmSoF64BbRFmk9C3p1VsBJ

Standalone/Core/src/VoxelForgeCore.cpp

@@ -909,6 +909,16 @@ namespace VoxelForgeCore
         return ok;
     }
 
+    bool World::needsTileRegion(const TileKey& key) const
+    {
+        if(!impl_||!impl_->owner||!impl_->shape||!impl_->shape->IsBuilt()
+            ||key.lod<0||key.lod>=impl_->configuration.fullResolutionLodCount)
+            return false;
+        if(CoveredByBaseShape(impl_->buildRequest,impl_->configuration,key))return false;
+        std::shared_lock lock(impl_->tileRegionMutex);
+        return !impl_->tileRegions.contains(key);
+    }
+
     bool World::compileTileRegion(std::span<const TileKey> tiles,std::uint32_t workerLimit,
                                   const JobScheduler& scheduler,std::string& error,
                                   std::size_t* outCompiledTiles,