refactor(generator): delete the pre-operator-stack archetype functions

With the operator stack as the only density path, the per-archetype
density functions have no caller left in the game:

- Delete UVoxelGenerator::GetSlabDensity, GetMazeDensity,
  GetSurfaceDensity, SurfaceDensityFromColumn, GetVerticalShaftDensity
  and GetFloatingIslandDensity (~1000 lines) with their declarations.
- Drop GetDensityWithParams' OutWarpedPosition out-parameter; only
  the native GetDensityAt branch removed in the previous commit used it.
- OpStack Slab/Maze/VerticalShaft/FloatingIsland and SurfaceHeight
  tests: remove the stack-vs-legacy comparisons; keep decomposition,
  finiteness, liveness, window-invariance, cache-cost and box-verdict
  checks (Shaft cache timing now measures the stack source).
- PassageLandsInOpenSpace: the source-only footing probe evaluates
  the archetype stack built without a strate manager instead of the
  deleted functions.
- Comments that cited the deleted functions now cite the operators
  (FSlabVoidSource, FLatticeCorridorSource, FIslandBlobSource, ...).
  GSurfColCache is documented as ClassifyTile's column cache.

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

Source/VoxelForge/Private/Tests/VoxelForgeTestFixture.h

@@ -230,8 +230,8 @@ namespace VoxelForgeTest
     /**
      * A spread of world sample points that deliberately crosses chunk boundaries, strate
      * boundaries and bedrock gaps — the exact conditions under which a per-chunk cache with a
-     * missing key input produces a wrong answer. Integer XY on purpose: that is the branch
-     * GetDensityAt's T1.a column cache actually takes (fractional XY bypasses the cache).
+     * missing key input produces a wrong answer. Integer XY on purpose: that is the lattice the
+     * mesher samples, and the one the per-column memos are keyed on.
      */
     inline void BuildSamplePoints(const FTestWorld& World, int32 Count, int32 Seed,
                                   TArray<FVector>& OutPoints)