fix: call PrepareChunk and guard the degenerate strate on the wired path

Read the step-3 wiring against the equivalence test before spending a build.
The symbols all line up; the two PATHS did not.

- GetDensityAt never called FVoxelOpStack::PrepareChunk, though the test does.
  All seven concrete bodies are empty today so behaviour is unchanged — which is
  the reason to fix it now: the first op to hoist real per-chunk work would have
  been green in test and silently wrong in game. Builds an FVoxelOpContext in the
  same refetch block (chunk, seed, layout version, strate Z bounds). Step stays 1;
  GetDensityAt does not know the mesher's sampling step (T2.b).

- GetMazeDensity early-outs to air on a degenerate strate (height <= 0) and the
  stack has no such early-out by design. Unguarded that is air on one path and
  spine/seal-of-a-zero-height-band on the other, so the wired path now falls back
  to the switch there — the reference behaviour is the behaviour.

Docs: VoxelDensityOpStack.h's banner still claimed nothing here feeds the game,
and CODEMAP 3.2d repeated it. Both now state what is wired (GetDensityAt) and
what is not (ClassifyTile, hand-written guards, Phase 2), with C10's never-compare
rule at the point of use. CODEMAP gains UsesOperatorStackForChunk and
bUseOperatorStack rows, and BuildMazeStack's degenerate-strate precondition.

UNVERIFIED: not compiled.

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

CODEMAP.md

@@ -118,8 +118,13 @@ stack share ONE copy. `VoxelGenerator.cpp` keeps same-named `static FORCEINLINE`
 **Convention: INTERNAL (positive = solid).**
 
 ### 3.2d Operator stack — `Public/VoxelDensityOpStack.h` + `Private/VoxelDensityOpStack.cpp`
-⚠️ **Feeds nothing yet.** `GetDensityAt`/`ClassifyTile` are untouched; the archetype `switch` is
-still the only production path. Exercised solely by `VoxelForge.OpStack.MazeEquivalence`.
+⚠️ **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.
+**`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).
 
 | Symbol | Role | Notes |
 |--------|------|-------|
@@ -129,7 +134,7 @@ still the only production path. Exercised solely by `VoxelForge.OpStack.MazeEqui
 | `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::BuildMazeStack` | — | The 7-op Maze stack. If this ever becomes one op, the refactor failed its own test (§2.5). |
+| `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
 `VoxelChunk.h` (the old `FVoxelChunk` coord wrapper) was DELETED — dead since the tile
@@ -269,7 +274,9 @@ Header is rich with inline docs. Two namespaces + a per-chunk cache system.
 `TransitionType`(79)/`TransitionBlendChunks`(89), `GeneratorType`(102),
 `GenerationParams`(113), `SlabParams`(124), `Biomes[]`+`BiomeMapParams` (the biome list +
 field tuning — empty ⇒ unchanged world, §8.14), `TerrainOperations`(147), visuals/fog/light,
-content lists, audio, `GameplayTags`(223). EditConditions show/hide param groups by generator type.
+content lists, audio, `GameplayTags`(223), `bUseOperatorStack` (the OPSTACK A/B opt-in — only bites
+if the archetype is in `UsesOperatorStackForChunk`'s ported list). EditConditions show/hide param
+groups by generator type.
 
 **`Public/VoxelStrateManager.h` + `.cpp`** — `UVoxelStrateManager : UObject` (h:108).
 Maps depth→strate at runtime; owns passages.
@@ -286,6 +293,7 @@ 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. |
 | `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). |

OPSTACK-PROGRESS.md

@@ -836,3 +836,43 @@ undiscovered, while it was worked out the slow way. Worth reading before clearin
 **Next single action unchanged:** build Phase 1 step 3, then the visual A/B on a Maze strate.
 
 ---
+
+## 2026-07-27 — fresh context: two gaps closed in the step-3 wiring BEFORE the build.
+
+Resumed from `OPSTACK-HANDOFF.md`. Read the step-3 diff against the test path instead of taking
+"not compiled" as "nothing to check first" — the symbols all line up (`BuildMazeStack`'s five params
+match, `Seed`/`OriginSpineRadius` are generator members, `FVoxelOpContext` comes in transitively via
+`VoxelDensityOp.h`), but **the production path and the test path did not agree on two things.**
+
+**1. `PrepareChunk` was never called in production.** The test calls it (`…MazeTest.cpp:133`);
+`GetDensityAt` did not. All seven concrete `PrepareChunk` bodies are empty today, so this changes
+**nothing** now — which is exactly why it was worth fixing before it could bite. The first op that
+hoists real per-chunk work would have been **green in test and silently wrong in game**, and that
+class of bug is expensive to find precisely because the test says yes. `GetDensityAt` now builds an
+`FVoxelOpContext` (chunk, seed, layout version, strate Z bounds) in the same refetch block and calls
+`PrepareChunk` on it. `Step` stays 1 — `GetDensityAt` genuinely does not know the mesher's sampling
+step (T2.b contract); noted rather than guessed.
+
+**2. The degenerate-strate early-out had no counterpart.** `GetMazeDensity` opens with
+`if (StrateHeight <= 0.0f) return 1.0f;` — air. The stack has no such early-out **by design** (the
+test asserts this and refuses to run on a degenerate strate). Unguarded, a zero-height Maze strate
+would give **air on one path and whatever the spine/seal ops make of a zero-height band on the
+other**. `GetDensityAt` now falls back to the `switch` in that case, so the reference behaviour is
+the behaviour. Reachability is not the point — the archetype guard exists, so its port needs one.
+
+**Docs corrected, since both were now actively false:**
+- `VoxelDensityOpStack.h`'s banner still said "NOTHING HERE FEEDS THE GAME". It does feed the game
+  now, behind the opt-in. Rewritten to say exactly what is wired (`GetDensityAt`) and what is not
+  (`ClassifyTile` — still hand-written guards, that is Phase 2), plus the §C10 "never compare the
+  two paths" rule at the point of use.
+- `CODEMAP §3.2d` had the same stale claim; `§3` gained rows for `UsesOperatorStackForChunk` and
+  `bUseOperatorStack`, and the `BuildMazeStack` row now carries the degenerate-strate precondition.
+
+**UNVERIFIED:** still not compiled — that is Jahni's call and it is the immediate next action.
+Error spots unchanged, plus one: `FVoxelOpContext` is aggregate-initialised field-by-field, so a
+field rename would show up here.
+
+**Next single action unchanged:** build, then the visual A/B on a Maze strate (§2.6's bar —
+recognisably the same maze, judged on a screenshot). Then Phase 2, starting with the §3.1 question.
+
+---

Source/VoxelForge/Private/VoxelGenerator.cpp

@@ -537,9 +537,28 @@ float UVoxelGenerator::GetDensityAt(float WorldX, float WorldY, float WorldZ) co
             if (CP_UseOpStack)
             {
                 CP_OpStack = FVoxelOpStack();   // move-assign : libère l'ancienne pile
+                FVoxelOpContext OpCtx;
+                OpCtx.ChunkCoord    = ChunkCoord;
+                OpCtx.Seed          = (uint32)Seed;
+                OpCtx.LayoutVersion = LayoutVersion;
+
                 switch (CP_GenType)
                 {
                 case ECaveGeneratorType::Maze:
+                    // GARDE DE STRATE DÉGÉNÉRÉE : GetMazeDensity court-circuite sur `return 1.0f`
+                    // (= air) quand la hauteur est nulle ou négative ; cette garde appartient à la
+                    // fonction d'archétype et la pile n'en a pas, par conception. Sans ce test, une
+                    // strate dégénérée donnerait de l'air sur un chemin et de la géométrie sur
+                    // l'autre. On retombe sur le `switch`, qui EST le comportement de référence.
+                    // Degenerate-strate guard: the archetype early-outs to air, the stack has no
+                    // such early-out by design. Fall back to the switch, which is the reference.
+                    if (CP_Maze.StrateTopWorldZ - CP_Maze.StrateBottomWorldZ <= 0.0f)
+                    {
+                        CP_UseOpStack = false;
+                        break;
+                    }
+                    OpCtx.StrateTopWorldZ    = CP_Maze.StrateTopWorldZ;
+                    OpCtx.StrateBottomWorldZ = CP_Maze.StrateBottomWorldZ;
                     VoxelDensityOps::BuildMazeStack(CP_OpStack, CP_Maze, Seed,
                                                     OriginSpineRadius, StrateManager);
                     break;
@@ -550,6 +569,18 @@ float UVoxelGenerator::GetDensityAt(float WorldX, float WorldY, float WorldZ) co
                     CP_UseOpStack = false;
                     break;
                 }
+
+                // Le test appelle PrepareChunk, pas la production : c'est exactement la divergence
+                // qui rend un opérateur vert en test et faux en jeu. Les sept `PrepareChunk`
+                // actuels sont vides, donc ceci ne change RIEN aujourd'hui — c'est le point : le
+                // premier opérateur qui hisse vraiment du travail par chunk doit trouver l'appel
+                // déjà là. `Step` reste 1 : GetDensityAt ne connaît pas le pas d'échantillonnage
+                // du mesher (voir le contrat T2.b dans VoxelDensityOp.h).
+                // The test calls PrepareChunk and production did not — the exact divergence that
+                // makes an op green in test and wrong in game. All seven bodies are empty today,
+                // which is the point: the first op that hoists real per-chunk work must find the
+                // call already here.
+                if (CP_UseOpStack) { CP_OpStack.PrepareChunk(OpCtx); }
             }
         }
 

Source/VoxelForge/Public/VoxelDensityOpStack.h

@@ -2,13 +2,21 @@
 // La PILE : un conteneur ordonné d'opérateurs, plus les fabriques d'opérateurs concrets.
 // The STACK: an ordered container of operators, plus the concrete-operator factories.
 //
-// ⚠️ RIEN ICI N'ALIMENTE LE JEU. `UVoxelGenerator::GetDensityAt` et `ClassifyTile` ne sont pas
-// touchés ; le `switch` par archétype reste le seul chemin de production. Cette pile est construite
-// et exercée UNIQUEMENT par le test `VoxelForge.OpStack.MazeEquivalence`, qui la compare point par
-// point à `GetMazeDensity`. Le branchement attend un build vert (OPSTACK-PLAN §4, Phase 1, point 3).
+// ⚠️ CECI ALIMENTE LE JEU, MAIS SEULEMENT SUR OPT-IN (depuis OPSTACK-PLAN §4, Phase 1, point 3).
+// `UVoxelGenerator::GetDensityAt` construit la pile par chunk et l'évalue à la place du `switch`
+// UNIQUEMENT quand `UVoxelStrateManager::UsesOperatorStackForChunk` rend true — c.-à-d. quand la
+// strate a coché `bUseOperatorStack` ET que son archétype figure dans la liste des portés (Maze
+// seul aujourd'hui). Toute autre strate passe encore par le `switch`, inchangé.
+// `ClassifyTile` n'est PAS branché : il utilise toujours ses gardes écrites à la main, pas
+// `ClassifyBox`. C'est la Phase 2.
 //
-// NOTHING HERE FEEDS THE GAME. GetDensityAt and ClassifyTile are untouched; the archetype switch is
-// still the only production path. This stack is built and exercised only by the equivalence test.
+// THIS FEEDS THE GAME, BUT ONLY BEHIND AN OPT-IN. GetDensityAt builds the stack per chunk and
+// evaluates it instead of the switch only when UsesOperatorStackForChunk returns true (strate
+// ticked bUseOperatorStack AND its archetype is ported — Maze only, today). ClassifyTile is NOT
+// wired: it still uses its hand-written guards rather than ClassifyBox. That is Phase 2.
+//
+// ⛔ NE JAMAIS faire tourner les deux chemins dans le même monde, ni les comparer pour l'égalité :
+// le résidu de ~1 ULP est INHÉRENT et documenté (AUDIT-2026-07 §C10). La barre est visuelle (§2.6).
 //
 // POURQUOI CETTE FORME / WHY THIS SHAPE
 // La question à laquelle la Phase 1 doit répondre n'est pas « est-ce que ça marche ? » mais