refactor(opstack): the operator stack is the only density path

Every strate now generates through its operator stack: authored
definitions, cooked-season entries (recipe or native) and editor
composer candidates alike. The per-strate opt-in and the archetype
switch it guarded are gone.

- Remove UVoxelStrateDefinition::bUseOperatorStack and the season
  wiring that set it; UsesOperatorStackForChunk now only says whether
  the slot has a known archetype.
- GetDensityAt always builds a stack (composer, then recipe, then the
  native archetype stack); a degenerate zero-height strate stays air.
- Remove the startup opt-in warning, the operator_stack startup-trace
  field and the four "Cave Bail Not Op Stack" stats (the remaining
  bail is counted by Cave Bail No Stack).
- Explore commandlet: drop -opstack and the operator_stack fields;
  VoxelForgeTest.ps1 no longer passes -opstack=1.
- Tests: FTestWorld::Build loses its bool parameter; legacy-vs-stack
  comparisons (MazeSeam, VerticalShaftSeam, TerrainDetailLiveness,
  WorldEdgeSeal, TilePostReach) collapse to the single stack path.

This changes terrain for any strate asset that did not already have
the opt-in ticked.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01URDGLsu7c4nunqNFPJEFPU

Tools/VoxelForgeTest.ps1

@@ -633,7 +633,6 @@ function New-CommandletArguments([string]$CommandletOut, [string]$Modes, [bool]$
     [void]$args.Add('-seed=0')
     [void]$args.Add('-archetype=TunnelNetwork')
     [void]$args.Add("-slot=$Slot")
-    [void]$args.Add('-opstack=1')
     [void]$args.Add(('-modes="{0}"' -f $Modes))
     [void]$args.Add('-blockearlyout=0')
     [void]$args.Add('-densitygridreuse=1')