feat: Phase 2 first port — FlatPlain + CrystalChamber collapse into one op

Jahni closed OPSTACK-DECOMPOSITION 3.1: the slab noise Z term was not
intentional character. Phase 1 also closed — the visual A/B on Maze passed.

Two changes, deliberately together, kept attributable by the test:

1. Design: GetSlabDensity's floor and ceiling noise lose their Z terms. A floor
   height no longer depends on the altitude you sample it from. The ceiling keeps
   its + 3000.0f, which is a decorrelation offset, not a Z term. The world
   re-tunes once — a different slice of the noise field, not a worse one.

2. Refactor: the now-XY-pure function ports to FSlabVoidSource + FGridColumnMod
   plus the three structural ops. BuildSlabStack has NO branch on archetype
   because GetSlabDensity never had one — CrystalChamber is FlatPlain with a
   bigger CeilingRoughness. 8 archetypes -> 7.

SlabEquivalence compares against the reference AS IT IS NOW and runs the whole
battery on both slots, so green means the port is a pure refactor and any visual
delta is attributable to the Z-term removal alone. The attribution comes from the
test, not from splitting it across two builds.

The payoff 3.1 was actually about: FSlabVoidSource::ClassifyBox is exact and needs
no sampling. FBM is contractually [-1,1], so both surfaces live in Z bands with
known bounds — a tile below the floor band is provably solid, a tile between the
bands provably air. ClassifyTile proves zero tiles for these archetypes today.
FGridColumnMod answers Identity when no column reaches the box, which is what lets
the source's AllAir verdict survive the fold.

UNVERIFIED: not compiled.

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

CODEMAP.md

@@ -121,7 +121,8 @@ stack share ONE copy. `VoxelGenerator.cpp` keeps same-named `static FORCEINLINE`
 ⚠️ **Feeds the game, behind a per-strate opt-in** (Phase 1 step 3). `GetDensityAt` builds the stack
 in its per-chunk refetch block and evaluates it *instead of* the `switch` only when
 `UVoxelStrateManager::UsesOperatorStackForChunk` says so — strate ticked `bUseOperatorStack` **and**
-archetype in the ported list (**Maze only**). Everything else still takes the `switch`, unchanged.
+archetype in the ported list (**Maze, FlatPlain, CrystalChamber**). Everything else still takes the
+`switch`, unchanged.
 **`ClassifyTile` is NOT wired** — still hand-written guards, not `ClassifyBox`. That is Phase 2.
 ⛔ Never run both paths in one world, and never compare them for equality: the ~1 ULP residue is
 inherent (AUDIT §C10). The acceptance bar is visual (OPSTACK-PLAN §2.6).
@@ -134,6 +135,9 @@ inherent (AUDIT §C10). The acceptance bar is visual (OPSTACK-PLAN §2.6).
 | `VoxelDensityOps::MakeLatticeCorridorSource` | 1 | Maze corridors, SDF channel. Edge identity = `hash(lower node, axis)` ⇒ adjacent chunks cannot disagree (AUDIT §6.4's preferred pattern). Its `EffectOverBox` answers for the source+carve **pair** (Phase 1 simplification) so it must be told the downstream `ExtraReach`. |
 | `VoxelDensityOps::MakeSdfRoughnessMod` | 3 | Wall roughness in **SDF** space (Maze/Shafts/Islands variant). TunnelNetwork's density-space roughness is a **different op** — see OPSTACK-DECOMPOSITION §1. |
 | `VoxelDensityOps::MakeSdfCarve` | 2 | SDF → density carve. The same six lines currently copied in three archetypes. |
+| `VoxelDensityOps::MakeSlabVoidSource` | 1 | Floor surface + ceiling surface → void field. **XY-pure** since §3.1, which is what gives it an **exact `ClassifyBox` with no sampling**: FBM's `[-1,1]` contract bounds both surfaces into known Z bands. Serves FlatPlain **and** CrystalChamber. |
+| `VoxelDensityOps::MakeGridColumnMod` | 3 | Infinite-height cylinders on a world grid, 3×3 cell memo. Adds solid only ⇒ `FillOnly` when a column reaches the box, `Identity` otherwise — and that `Identity` is what lets the source's `AllAir` verdict survive. |
+| `VoxelDensityOps::BuildSlabStack` | — | 5 ops, **no branch on archetype**: FlatPlain and CrystalChamber differ only in defaults, exactly as `GetSlabDensity` already had it. 8 archetypes → 7. |
 | `VoxelDensityOps::BuildMazeStack` | — | The 7-op Maze stack. If this ever becomes one op, the refactor failed its own test (§2.5). Callers must skip it on a **degenerate strate** (top−bottom ≤ 0): `GetMazeDensity` early-outs to air there and the stack has no such early-out by design — `GetDensityAt` falls back to the `switch`. |
 
 ### 3.3 Chunk identity
@@ -293,7 +297,7 @@ Maps depth→strate at runtime; owns passages.
 | `GetLayoutVersion` | h:161 (inline) | Layout/passage generation counter (= `PassagesVersion`, bumped by every `Initialize`). Hot-path callers key `thread_local` memos on it (strate-index memo in `GetDensityWithParams`, passage shortlist) so editor rebuilds never serve stale data. |
 | `GetStrateForChunk` | 466 | Chunk → definition. |
 | `GetGeneratorTypeForChunk` | 476 | Chunk → generator type. |
-| `UsesOperatorStackForChunk` | 559 | Chunk → should `GetDensityAt` take the operator stack? `bUseOperatorStack` on the definition **AND** archetype in the ported list. **That list is written down here and nowhere else** — an unported archetype ignores the flag, so ticking the box anywhere is harmless. Add a row here when you port one. |
+| `UsesOperatorStackForChunk` | 559 | Chunk → should `GetDensityAt` take the operator stack? `bUseOperatorStack` on the definition **AND** archetype in the ported list (Maze, FlatPlain, CrystalChamber). **That list is written down here and nowhere else** — an unported archetype ignores the flag, so ticking the box anywhere is harmless. Add a row here when you port one. |
 | `GetSlabParamsForChunk` | 490 | Slab params with runtime Z bounds (no blend — slabs use Hard). |
 | `GetBiomeContextForChunk` | — | Flatten the strate's `Biomes[]` + `BiomeMapParams` into a POD `FBiomeContext` for the biome field. Empty ⇒ biomes disabled. §8.14. |
 | `GetGenerationParams` | 515 | **Blended** TunnelNetwork params (handles Gradient/Hard/Interleaved transitions). |
@@ -376,6 +380,7 @@ The plugin's first tests (`OPSTACK-PLAN.md` Phase 0.5). Run them from the editor
 | `VoxelForgeClassifyTileTest.cpp` | `VoxelForge.Determinism.ClassifyTileSoundness` | Scans for a non-`Mixed` verdict, then brute-forces the exact mesher lattice (`g ∈ [-1, Cells+1]`). **A false verdict is an invisible, collisionless hole** — T1.d v1 was reverted for exactly this. Errors out rather than passing if it found nothing to check. |
 | `VoxelForgeDiffLayerTest.cpp` | `VoxelForge.Determinism.DiffLayerContention` | N readers running the worker call mix while the game thread writes and `Clear()`s. Survival + monotonic `ModsVersion`. |
 | `VoxelForgeClassifyTileTest.cpp` | `VoxelForge.OpStack.BoxVerdictFold` | Pure-logic walk of the fold in `VoxelDensityOp.h`, case by case — including the seal-forces-AllSolid case that justifies `ClassifyBox` existing. Also the only `.cpp` that includes the op header, so the build actually sees it. |
+| `VoxelForgeOpStackSlabTest.cpp` | `VoxelForge.OpStack.SlabEquivalence` | **Phase 2's first port.** The same 5-op slab stack vs `GetSlabDensity` over 20k points, run twice — FlatPlain **and** CrystalChamber — which is what demonstrates the two archetypes really are one op. Plus window-invariance and box-verdict brute force. Compares against the reference **as it is now** (post Z-term removal), so green = pure refactor and any visual delta is attributable to §3.1 alone. |
 | `VoxelForgeOpStackMazeTest.cpp` | `VoxelForge.OpStack.MazeEquivalence` | **Phase 1's load-bearing test.** The 7-op Maze stack vs `GetMazeDensity` over 20k points (aiming for bit-identity; a side-of-iso disagreement is the hard fail), plus purity across workers and brute force on every box verdict the stack emits. Reports how many tiles the stack can prove uniform — today's `ClassifyTile` proves **zero** for any cave archetype. |
 
 ## 4. The density pipeline (most-edited hot path)