wip(assembler): round L37 -- FI mouth carve-fade fix, no raised bridge (style 0); research R1

The passage mouth fade multiplied the signed carve distance by zero, which turned native rock into an
open surface (the reason earlier rounds stamped walk pads at mouths); it now blends between the
uncarved field and the full signed carve. The FI style-0 route followed a lifted grade envelope over
the crater (the "bridge"); it now runs on the rim and follows the native surface. New built-shape and
crease audits with thresholds frozen before the fix (L36 and box controls fail). 24/24 passage rows,
30/30 determinism, 16/16 invariants, 18/18 interface/ring. Unreal: one recipe setter (uncompiled).
Research R1 (read-only, web): ranked proposals for natural walkable mouths, floor-limited carve first.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016dhxM43dnWvUu2B5Ph1iG1

Standalone/Source/WorldAssemblyPrototype.h

@@ -743,6 +743,7 @@ namespace StandaloneWorldAssemblyPrototype
         const FStratePassageConfig& passage=islandSlot->Definition->PassageConfig;
         recipe.seed=static_cast<std::uint32_t>(owner.Seed);
         recipe.geometryVersion=8;
+        recipe.terrainFollowingRoute=static_cast<std::uint32_t>(passage.Style)==0;
         recipe.ownerVoxelSizeMicrons=owner.OwnerVoxelSizeMicrons;
         recipe.coreVoxelSizeMicrons=owner.CoreVoxelSizeMicrons;
         recipe.ownerToCoreScaleNumerator=owner.OwnerVoxelSizeMicrons/static_cast<std::uint32_t>(divisor);