feat: port FloatingIslands — the stack that runs backwards

6 of 8 archetypes ported. This one starts from VOID and FILLS where the
other four start from ROCK and CARVE, which is what it was worth doing:
neither end of the pile needed a new operator, only the opposite sign.

  FConstantRockSource -> FConstantFieldSource(+/-Base)   AllSolid <-> AllAir
  FSdfCarveOp         -> FSdfConvertOp(Sign = +/-1)      carve    <-> fill
  FSdfRoughnessMod                                       4th archetype, unchanged

Only the island blob source is new. Multiplying by +/-1 is exact in
IEEE-754, so the three already-green ports are bit-for-bit untouched.

ClassifyBox can return AllAir for the first time in the plugin, and an
island strate is by construction mostly empty — the test counts AllSolid
and AllAir separately so an aggregate cannot hide whether that fired.

Two bounds that would have been holes if assumed rather than derived:
the island bound is one-sided (a hairline thread of matter hangs below
each island down its axis, so only the TOP may reject), and the domain
warp displaces X and Y independently, so the pad needs WarpAmp*sqrt(2).

Also: AUDIT C1 was NOT closed. The 2026-07-27 sweep matched `SeedF * K`
and this archetype's warp spells it `(float)S * K`, so one site survived
— at seed 2e9 the warp flattens and every island snaps back to a perfect
circle. Fixed in both paths in one pass so the equivalence test stays a
valid oracle. Expect island silhouettes to change at large seeds.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Source/VoxelForge/Private/VoxelStrateManager.cpp

@@ -585,6 +585,11 @@ bool UVoxelStrateManager::UsesOperatorStackForChunk(const FIntVector& ChunkCoord
 
     case ECaveGeneratorType::VerticalShafts:  return true;   // Phase 2 — 3 ops repris de Maze tels quels
 
+    case ECaveGeneratorType::FloatingIslands:
+        // Phase 2 — la pile qui tourne à l'ENVERS : source de VIDE + fill, au lieu de source de ROC
+        // + carve, avec les MÊMES opérateurs au signe près. 6 des 8 portés.
+        return true;
+
     default:                                  return false;
     }
 }