refactor(sheets): remove the far sheet ring from the runtime

The owner dropped the far sheet ring. Remove it from the streaming path:
- UVoxelMarchingCubesMesher::GenerateSheetMesh (declaration + definition);
- sheet-tile branches in LoadTile / GenerateTileResult / SyncRemeshTile and
  the bSheetTile / SheetChunkZ / Hole* parameters;
- the SheetHole* XY-hole state and its per-crossing re-queue block;
- VF_MaxMarchingCubesLevel (equal to the clamped MaxClipLevel without sheets);
- the sheet-only band arming and the 'sheet' field of the desired-level
  startup-trace events (no tool reads it).

AVoxelWorld now always passes bFarSheetRing=false to the tile selector, and
the descendant index covers levels up to the MaxClipLevel clamp (8) instead
of the old sheet maximum (12); no tile above 8 can exist any more.

Kept on purpose: UVoxelSettings::bFarSheetRing / FarSheetSpanLevels
(UPROPERTYs, now marked unused), the voxel.TestFarSheetRing switch and the
far_sheet_ring / sheet_tile fields the test harness compares (sheet_tile is
always false), and the selector's sheet parameters in
VoxelClipmapDesiredTiles.h, which two automation tests still exercise.

Source/VoxelForge/Public/VoxelSettings.h

@@ -183,22 +183,13 @@ public:
 	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Voxel|Clipmap", meta = (ClampMin = "0"))
 	int32 RenderDistanceChunks = 768;
 
-	// F18 — the render-distance ring streams per-surface SHEETS instead of MC tiles: in an open
-	// strate the far field is exactly two heightfields (TerrainZ + sky-cap CeilSurf, both already
-	// computed per column), so each far tile becomes two displaced grids (ground polygroup 0 /
-	// cap polygroup 1 — same materials), ~3-6× cheaper to generate and far fewer components (one
-	// sheet spans 2^FarSheetSpanLevels MC-tile footprints per axis). Non-open strates produce
-	// empty sheets (their far ring was enclosed rock anyway). Carved features (passages, chasms,
-	// spine) don't show at sheet distance. Needs the strate band armed (StrateContentCutMinLevel
-	// active); in the inter-strate gap the sheet ring blanks until you land. The ship default is OFF:
-	// the ring stays real MC tiles at level MaxClipLevel (pre-F18 behaviour). Only matters when
-	// RenderDistanceChunks > 0.
+	// UNUSED: the far sheet ring was removed from the runtime; the render-distance ring is always
+	// marching-cubes tiles at MaxClipLevel. Kept only so saved assets and the test harness's
+	// `voxel.TestFarSheetRing` switch still load; it has no effect.
 	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Voxel|Clipmap")
 	bool bFarSheetRing = false;
 
-	// Sheet tile size = MaxClipLevel + this many levels (2 → one sheet covers 4×4 MC-tile
-	// footprints → ~16× fewer far components). Sampling density stays that of the MaxClipLevel
-	// MC ring (cell count grows instead), capped at 128 cells/axis (beyond, cells coarsen).
+	// UNUSED (see bFarSheetRing). Kept so saved assets still load; it has no effect.
 	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Voxel|Clipmap", meta = (ClampMin = "1", ClampMax = "4", EditCondition = "bFarSheetRing"))
 	int32 FarSheetSpanLevels = 2;