feat: TunnelNetwork stage A — the SDF spine, wrapping BuildChunkCache

The last archetype is ~1080 lines with 13 detail modifiers, a two-region
cache and a per-room op override. Porting it whole before anything can be
verified is ~600 unverified lines on top of ~200 — the pattern this
refactor has dodged six times. So: three stages.

Stage A = vertical scale, base rock, cave warp, room graph (+ pits and
chimneys), carve, worms, structural post. 6 ops. It is verifiable NOW
because every detail modifier is amplitude-gated and FStrateGenerationParams
already defaults them all to zero — zeroing SurfaceRoughness sends the
ORIGINAL down exactly the path stage A ported.

TunnelNetwork stays OFF in UsesOperatorStackForChunk until stage C.

The decision that matters: FRoomGraphSource CALLS BuildChunkCache and
EvaluateSDFCached rather than transcribing them. That is where §8.4's
two-region window-invariance discipline lives; a transcription would fork
it, and the fork would be "validated" by a test comparing it to the
original. Only the ~60 lines of glue are transcribed.

FRAME ops are retired. All three candidates are now ported and none needed
one: CaveWarp's scope is exactly one operator (pits/chimneys read unwarped
coords), VerticalScale is a one-line pure function, and the island warp was
already local. Not missing infrastructure — one idea seen three times from
a distance.

Also: check 3 was going to compare two interleaved param sets against the
original, which would have FAILED — the original's SDF cache key has no
params, so it serves B the rooms it built for A. Comparing there measures
its bug, not the port. Rewritten against each stack evaluated alone. The
same reasoning suggests a live production staleness across Gradient
transitions; filed in AUDIT §C2 as SUSPECTED with the check that would
confirm it, since it rests on a premise I have not verified.

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

Source/VoxelForge/Public/VoxelDensityOpStack.h

@@ -193,8 +193,13 @@ namespace VoxelDensityOps
                                                                    int32 BaseOctaves, float ApplyWithin);
 
     /** Rôle 2 — conversion SDF → densité : creuse de l'air là où le SDF est à l'intérieur.
-     *  Les six mêmes lignes apparaissent aujourd'hui dans TunnelNetwork, Maze et VerticalShafts. */
-    VOXELFORGE_API TUniquePtr<IVoxelDensityOp> MakeSdfCarve(float Blend, float BaseDensity);
+     *  Les six mêmes lignes apparaissent aujourd'hui dans TunnelNetwork, Maze et VerticalShafts.
+     *  @param MinDivisor  plancher du diviseur `Blend·2`. **TunnelNetwork passe 1.0** (son original
+     *                     écrit `FMath::Max(SDFBlendRadius·2, 1)`) ; Maze/Shafts laissent 0, où
+     *                     `Max(x,0) == x` exactement. Les deux formules divergent si `Blend·2 < 1`,
+     *                     donc ce paramètre est une vraie différence, pas une précaution. */
+    VOXELFORGE_API TUniquePtr<IVoxelDensityOp> MakeSdfCarve(float Blend, float BaseDensity,
+                                                            float MinDivisor = 0.0f);
 
     /** Rôle 2 — la même conversion, signe opposé : REMPLIT du solide là où le SDF est à l'intérieur.
      *  C'est ce que fait FloatingIslands (`Density += Fill·Base·2`), et la multiplication par ±1
@@ -260,6 +265,24 @@ namespace VoxelDensityOps
                                                 int32 Seed, float SpineRadius,
                                                 const UVoxelStrateManager* StrateManager);
 
+    /**
+     * TunnelNetwork — **ÉTAPE A SUR TROIS, PILE INCOMPLÈTE** :
+     *   ConstantRock → RoomGraph(warp + pits + cheminées) → SdfCarve → Worms → [structural ×3]
+     *
+     * ⛔ NE PAS brancher cet archétype dans `UsesOperatorStackForChunk` avant l'étape C : les 13
+     * modificateurs de détail (4b–4h) et l'override d'op par salle ne sont pas portés, donc le monde
+     * y perdrait tout son détail. Le test compare avec ces amplitudes à zéro.
+     *
+     * ⚠️ `FRoomGraphSource` **APPELLE** `BuildChunkCache`/`EvaluateSDFCached`, il ne les transcrit
+     * pas : c'est là que vit la discipline d'invariance de fenêtre à deux régions (`ARCHITECTURE
+     * §8.4`), et en faire une copie serait le pire résultat possible pour un refactor dont le but est
+     * d'avoir UNE définition de chaque idée.
+     */
+    VOXELFORGE_API void BuildTunnelNetworkStack(FVoxelOpStack& OutStack,
+                                                const FStrateGenerationParams& P,
+                                                int32 Seed, float SpineRadius,
+                                                const UVoxelStrateManager* StrateManager);
+
     /**
      * FloatingIslands — 7 ops, et **la pile tourne à l'ENVERS** :
      *   ConstantVoid → IslandBlob → SdfRoughness → SdfFill → [structural post ×3]