unreal: parallel sign-edge repair + cheap SW route prefilter (L94); SW startup 2.75 -> 0.43 s

- Unreal requests Core's deterministic parallel sign-edge repair (UE ParallelFor, capped at
  10 workers); headless defaults to parallel (--serial-sign-edge-repair for A/B). SW
  sign_edge_distances 1,942 -> 37 ms, ShapeStage 2,156 -> 198 ms.
- SW fallback route planner checks the exact mouth-radius endpoint constraints before sampling
  terrain (58,480 of 78,408 candidates skip heightfield work); same 35 eligible, same chosen route.
  SerializeResolvedWorld 468 -> 115 ms; BeginPlay block 0.58 -> 0.22 s.
- Unreal SW: first admission 2.75 -> 0.43 s, LOD0-2 settled 5.20 -> 2.81 s; FI 3.40 -> 3.12 s.
- World identical: assembly/recipe/plan hashes unchanged; serial vs parallel per-tile hashes equal.
  Fast tier PASS; Linux UE host builds.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016dhxM43dnWvUu2B5Ph1iG1

Source/VoxelForge/Private/VoxelWorld.cpp

@@ -7812,6 +7812,7 @@ bool AVoxelWorld::InitializeVoxelForgeCoreWorld(FString& OutFailureReason)
     Request.workerLimit = static_cast<std::uint32_t>(FMath::Clamp(GetMaxConcurrentTasks(), 1, 16));
     Request.detail = VoxelForgeCore::DetailVariant::coherent;
     Request.includeFarWorld = false;
+    Request.parallelSignEdgeRepair = true;
 
     VoxelForgeCoreState = State;
     State->bSnapshotReady.store(true, std::memory_order_release);