streaming: split dense passage-junction grid fills; heavy-first order + far-parent deferral in Unreal (L85)

Tiles with passage-overlap score >= 9 split grid fill over up to 4 helper lanes (disjoint rows,
same per-sample order, join before MC) when workers are idle. FI junction tile fill 173 -> 99 ms;
estimated serial floor ~0.55-0.60 s. Readiness essentially unchanged (FI/TN LOD0-2 ~2.15 s @W12):
the limit is now aggregate work, not single tiles.
Unreal: LOD0-2 ordered by passage overlap before travel/distance (L80 modes kept); far parents
built after LOD0-1, LOD2 gated on them; helper budget via ParallelFor. Unreal null-RHI FI
arrival: LOD0-2 resident at 3.65 s.
All tile hashes identical to L84 (four whole views). 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

Standalone/Core/src/WorldAssemblerInternal.h

@@ -2653,6 +2653,9 @@ void Heightfield2D::setThreadColumnCaptureBounds(std::int64_t minimumXSubvoxels,
 void Heightfield2D::setThreadColumnCacheLegacyDirectMapped(bool enabled) noexcept
 { world_assembly_detail::HeightfieldColumnCacheLegacyDirectMapped=enabled; }
 
+bool Heightfield2D::threadColumnCacheLegacyDirectMapped() noexcept
+{ return world_assembly_detail::HeightfieldColumnCacheLegacyDirectMapped; }
+
 void Heightfield2D::setThreadColumnOctaveDrop(std::uint32_t octaveDrop) noexcept
 { world_assembly_detail::HeightfieldColumnOctaveDrop=octaveDrop; }