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>

OPSTACK-PLAN.md

@@ -4,9 +4,11 @@
 > *composable density pipeline*, so new world ideas become authoring instead of C++. Written
 > 2026-07-26 as a handoff for a future context — read this instead of re-deriving it.
 >
-> **Status (2026-07-27):** Phase 0.5 (tests) and the Phase 1 skeleton header are WRITTEN AND
-> COMMITTED but **NOT YET COMPILED**. No operator exists; `GetDensityAt`'s archetype `switch` is
-> untouched. Live state and the next action live in
+> **Status (2026-07-27):** **Phase 0.5 and Phase 1 are DONE and verified** — five green tests, Maze
+> decomposed into seven ops, wired into `GetDensityAt` behind `bUseOperatorStack`, and the visual
+> A/B passed (Jahni: *"pretty similar, if not entirely similar"*). **Phase 2 is in progress:**
+> FlatPlain + CrystalChamber are ported into ONE op (`BuildSlabStack`), their §3.1 Z term is gone,
+> and both are wired — **written, not yet compiled.** Live state and the next action live in
 > [OPSTACK-PROGRESS.md](OPSTACK-PROGRESS.md) — read its last entry first. The per-archetype
 > breakdown is in [OPSTACK-DECOMPOSITION.md](OPSTACK-DECOMPOSITION.md).
 >
@@ -356,8 +358,8 @@ Lipschitz-1 off a lattice), and it's the least-used archetype so a mistake is ch
 Port each archetype **the next time a feature makes you open it anyway**. The switch shrinks on its own.
 Suggested order when there's a free choice — cheapest and least risky first:
 
-`Maze` (P1) → `FlatPlain`/`CrystalChamber` (one op, two default sets — the first real win: two archetypes
-collapse into one) → `SurfaceWorld` (biggest payoff, biggest care: the T1.a column cache and the exact-
+✅ `Maze` (P1) → ✅ `FlatPlain`/`CrystalChamber` (one op, two default sets — the first real win: two
+archetypes collapse into one; **done 2026-07-27**, `BuildSlabStack`, 8 archetypes → 7) → `SurfaceWorld` (biggest payoff, biggest care: the T1.a column cache and the exact-
 lattice `ClassifyTile` bound must both survive) → `VerticalShafts` → `FloatingIslands` → `TunnelNetwork`
 (**last** — it owns `BuildChunkCache`'s two-region window-invariance discipline, §8.4, the most delicate
 code in the plugin).