feat(composer): lateral REGIONS -- built, measured, and gated OFF

Jahni's own idea: "archetype could even mix within the same strate at some
point, tunnel network leading to a big chamber". An archetype becomes a
spatial REGION inside a strate, not a label on it.

Built by extending the mechanism that already exists rather than inventing
one: the stack blends VERTICALLY between strates, so regions blend the same
way LATERALLY. Deterministic jittered Voronoi sites on a 256-voxel lattice,
1-3 regions per strate from (seed, strateIndex), 24-voxel symmetric blend
band, per-chunk lookup cache. Parameters are never blended -- a shaft's
ShaftSpacing means nothing to a slab -- only the resulting DENSITY is.
Structural posts stay global and unconditional, so an invented region
cannot switch off the primordial law.

  single-region path   BIT-IDENTICAL to legacy, 256 probes, native + recipe
  cross-boundary boxes 20 of 40, 199,800 voxels brute-forced, 0 VIOLATIONS
                       (every seam-spanning box correctly returned Mixed)
  blend-band cost      663 ns vs 338 ns per voxel, over 16.0% of voxels
                       ~1.15x field-weighted
  survival             16/16 valid, non-vacuous, largest share >= .50

⛔ AND IT IS GATED OFF, because the seam breaks the one law that matters:
arrival -> departure across a region boundary passes 9/16 seeds, against
11/16 same-region controls in the same harness. Density blending alone does
not guarantee a player can cross a seam. A strate that mixes archetypes and
strands you at the boundary is strictly worse than one that does not mix.

VF_LateralRegionsAreShippable() returns false and the test ASSERTS THE GATE
IS SHUT while the rate is imperfect -- so flipping it without fixing
connectivity turns the suite red by design. The rate is a warning, not an
assertion: it is the measured state of an off-by-default feature, not a
regression in a live world, and a permanently red suite stops being read,
which is how real failures get missed.

Everything that ships is still hard-guarded: single-region bit-identity,
zero cross-boundary box violations, and every shipping-path law test
untouched.

The fix is an explicit CROSS-SEAM CORRIDOR CONTRACT -- a design decision,
recorded rather than made unilaterally. Codex diagnosed this correctly and
declined to tune the threshold, which was the right call.

Build clean, 29 succeeded / 0 failed, run and verified by me.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VJiTcTSDNkKBtnNdiBtdaU

Source/VoxelForge/Private/VoxelWorld.cpp

@@ -194,6 +194,7 @@ namespace
         const FVoxelStrateComposerCandidate& Candidate,
         const UVoxelGenerator& Generator,
         const UVoxelStrateManager& StrateManager,
+        int32 TargetStrateIndex,
         int32 TargetTopChunkZ,
         int32 TargetBottomChunkZ,
         FVoxelOpStack& OutStack,
@@ -206,7 +207,24 @@ namespace
         VF_SetComposerRuntimeBounds(Params, TopWorldZ, BottomWorldZ);
 
         bool bBuilt = false;
-        if (Candidate.bStructureRoll)
+        if (Candidate.Regions.RegionCount > 1)
+        {
+            FVoxelStrateRegionManifest RegionManifest = Candidate.Regions;
+            VF_RekeyStrateRegionManifest(RegionManifest, Candidate.Seed,
+                                          TargetStrateIndex);
+            RegionManifest.StrateTopWorldZ = TopWorldZ;
+            RegionManifest.StrateBottomWorldZ = BottomWorldZ;
+            RegionManifest.bHasGlobalStructuralParams = true;
+            for (FVoxelStrateRegion& Region : RegionManifest.Regions)
+            {
+                VF_SetStrateArchetypeRuntimeBounds(Region.ArchetypeParams,
+                                                   TopWorldZ, BottomWorldZ);
+            }
+            bBuilt = VF_BuildStrateRegionStack(
+                RegionManifest, Generator.OriginSpineRadius, &StrateManager,
+                OutStack, OutContext, &OutError);
+        }
+        else if (Candidate.bStructureRoll)
         {
             bBuilt = VF_BuildStackFromRecipe(
                 Candidate.Recipe, Params, Generator.Seed, Generator.OriginSpineRadius,
@@ -573,7 +591,25 @@ void AVoxelWorld::ApplyComposerCandidate()
     FVoxelOpContext PreflightContext;
     FString PreflightError;
     bool bPreflightBuilt = false;
-    if (Candidate.bStructureRoll)
+    FVoxelStrateRegionManifest PreflightRegions;
+    if (Candidate.Regions.RegionCount > 1)
+    {
+        PreflightRegions = Candidate.Regions;
+        VF_RekeyStrateRegionManifest(PreflightRegions, Candidate.Seed,
+                                      ComposerTargetStrateIndex);
+        PreflightRegions.StrateTopWorldZ = TargetTopWorldZ;
+        PreflightRegions.StrateBottomWorldZ = TargetBottomWorldZ;
+        PreflightRegions.bHasGlobalStructuralParams = true;
+        for (FVoxelStrateRegion& Region : PreflightRegions.Regions)
+        {
+            VF_SetStrateArchetypeRuntimeBounds(Region.ArchetypeParams,
+                                               TargetTopWorldZ, TargetBottomWorldZ);
+        }
+        bPreflightBuilt = VF_BuildStrateRegionStack(
+            PreflightRegions, Generator->OriginSpineRadius, StrateManager,
+            PreflightStack, PreflightContext, &PreflightError);
+    }
+    else if (Candidate.bStructureRoll)
     {
         bPreflightBuilt = VF_BuildStackFromRecipe(
             Candidate.Recipe, PreflightParams, Generator->Seed, Generator->OriginSpineRadius,
@@ -615,7 +651,8 @@ void AVoxelWorld::ApplyComposerCandidate()
         if (!StrateManager->SetComposerOverrideForStrate(
             ComposerTargetStrateIndex, Candidate.Seed, Candidate.Archetype,
             Candidate.ArchetypeParams, Candidate.bStructureRoll,
-            Candidate.bStructureRoll ? &Candidate.Recipe : nullptr, ApplyError))
+            Candidate.bStructureRoll ? &Candidate.Recipe : nullptr, ApplyError,
+            Candidate.Regions.RegionCount > 1 ? &Candidate.Regions : nullptr))
         {
             UE_LOG(LogTemp, Error,
                 TEXT("[VoxelWorld] ApplyComposerCandidate: seed=%d index=%d was not applied: %s"),
@@ -638,7 +675,8 @@ void AVoxelWorld::ApplyComposerCandidate()
     FVoxelOpContext ReferenceContext;
     FString ReferenceError;
     const bool bReferenceBuilt = VF_BuildComposerReferenceStack(
-        Candidate, *Generator, *StrateManager, TargetTopChunkZ, TargetBottomChunkZ,
+        Candidate, *Generator, *StrateManager, ComposerTargetStrateIndex,
+        TargetTopChunkZ, TargetBottomChunkZ,
         ReferenceStack, ReferenceContext, ReferenceError);
 
     FString ComparableReason;