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>

ARCHITECTURE.md

@@ -33,6 +33,13 @@ StrateManager provides params per chunk via `GetMaze/Surface/VerticalShaft/Float
 On top of the archetype, an optional **biome** layer (§8.14) modulates terrain & content WITHIN a
 strate via a window-invariant XY field — currently wired into SurfaceWorld.
 
+⚠️ **The `switch` above is no longer the only density path.** 6 of the 8 archetypes (all but
+TunnelNetwork and Underwater) also exist as **operator stacks**, selected per strate by
+`bUseOperatorStack` and evaluated instead of the `switch`; each is bit-identical to the function in
+its row. The design lives in `OPSTACK-PLAN.md` / `OPSTACK-DECOMPOSITION.md`, the symbol index in
+`CODEMAP §3.2d` — not repeated here. What matters for *this* document: the archetype table describes
+what the world IS, and both paths compute it.
+
 ### 8.2 (0,0) spine & hybrid connections
 - `ApplyOriginSpine` (VoxelGenerator.cpp, static helper) carves a guaranteed open vertical
   column at XY (0,0) in every strate's **interior** (seals untouched). Radius =