passages: smooth 12 m lower-mouth radius taper; all four-strate passages pass (L68, v213)

The FI->TN wall beat (4.77 cm at 7.06 m) was the lower-mouth flare: the full mouth radius
was assigned to the penultimate control point, a one-segment radius jump. The shared route
builder now blends to the mouth radius over 12 m with a smoothstep. Wall ripple 1-2:
7.27 -> 1.62 cm. All three passages pass support/blocked/grade/ripple/naturalness; controls
detected. Fast tier 269/269 incl. the new-plan stage; W1/W8/W12 equal, Clang and GCC.
Hash pins: FI style 2 re-pinned at v213.

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

Standalone/Core/include/VoxelForgeCore/VoxelForgeCore.h

@@ -403,7 +403,7 @@ namespace VoxelForgeCore
     // Version 210 keeps bulk native-cave sampling on the scalar field path at strata edges.
     // Version 211 makes coordinate biasing in the shared cell hash unsigned, removing signed-overflow UB.
     // Version 212 balances generated entry routes against existing walk lanes and preserves their shared treads.
-    inline constexpr std::uint32_t WorldAssemblyEvaluatorVersion = 212;
+    inline constexpr std::uint32_t WorldAssemblyEvaluatorVersion = 213;
 
     enum class FieldChannel : std::uint32_t
     {

Standalone/Core/src/ResolvedLayoutInternal.h

@@ -3188,7 +3188,7 @@ bool PlanResolvedLayoutPassagesV1(ResolvedLayoutSnapshotV3& layout,std::int32_t
             for(std::size_t i=0;i<xy.size();++i)
             {
                 std::int64_t radius=(i<=1||i==lowerMouthIndex
-                    ||i+1>=xy.size()-1)?mouthRadius:midRadius;
+                    ||(i>lowerMouthIndex&&i+1>=xy.size()-1))?mouthRadius:midRadius;
                 if(surfaceGroundDeparture&&i<lowerMouthIndex)
                 {
                     const double t=std::clamp(static_cast<double>(cumulative[i])
@@ -3197,6 +3197,24 @@ bool PlanResolvedLayoutPassagesV1(ResolvedLayoutSnapshotV3& layout,std::int32_t
                     radius=surfacePortalRadius+static_cast<std::int64_t>(std::llround(
                         (midRadius-surfacePortalRadius)*smooth));
                 }
+                // Grow the lower bore into its mouth over fixed route distance. Assigning
+                // the full mouth radius to the penultimate point creates a segment-length
+                // flare whose cubic-detrended profile can read as a short-period wall beat.
+                if(i<=lowerMouthIndex)
+                {
+                    constexpr std::int64_t LowerMouthRadiusBlendLengthMetres=12;
+                    const std::uint64_t blendLength=static_cast<std::uint64_t>(
+                        std::max<std::int64_t>(LowerMouthRadiusBlendLengthMetres*qPerMetre,
+                            2*mouthRadius));
+                    const std::uint64_t distanceToLowerMouth=mouthHorizontal-cumulative[i];
+                    if(distanceToLowerMouth<blendLength)
+                    {
+                        const double t=static_cast<double>(distanceToLowerMouth)/blendLength;
+                        const double smooth=t*t*(3.0-2.0*t);
+                        radius=midRadius+static_cast<std::int64_t>(std::llround(
+                            (mouthRadius-midRadius)*(1.0-smooth)));
+                    }
+                }
                 std::int64_t playerZ=0;
                 if(i>lowerMouthIndex)
                     playerZ=lowerLandingPoints[i-lowerMouthIndex-1][2];