test(measure): the VerticalShafts disconnection is real, not a coarse lie

Our guard was one-directional. FullResolutionPathIsAir catches the false
POSITIVE -- coarse says joined, reality says walled -- which is the
dangerous direction for safety, so we built it first. The opposite error
was unguarded: a corridor narrower than SampleStep falls between sample
points and vanishes, so connected regions read as separate. That is
exactly what a network of narrow vertical shafts would fake at step 4.

Refinement sweep, radius held constant for the controls:

  step 4 / r256  NOT_CONNECTED  57 comp  largest .119     458,752 cells
  step 4 / r192  NOT_CONNECTED  32 comp  largest .220     258,048
  step 2 / r192  NOT_CONNECTED  27 comp  largest .220   2,064,384
  step 4 / r128  NOT_CONNECTED  20 comp  largest .291     114,688
  step 1 / r128  NOT_CONNECTED  11 comp  largest .303   7,340,032

Both fixed-radius pairs stayed negative, so neither the step nor the
radius explains it. At one-voxel sampling the arrival and departure
mouths are still in different air components.

VerticalShafts is a live primordial-law violation: a player descends into
it and cannot reach its exit.

NotConnected is renamed NotConnectedAtThisResolution and documents its own
limit at the declaration, so a bare negative can never again be read as
proof. CoarseLied names one direction; this names the other.

Sweep split into VoxelForge.Generation.StrateConnectivityRefinement so the
fast test stays fast. 4.4 s. The MaxCells refusal path was exercised and
still refuses.

Build clean, 19 succeeded / 0 warnings / 0 failed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Bp7FxoQcSoiuhxjuLr6aBM

Source/VoxelForge/Private/VoxelStrateMeasure.cpp

@@ -882,7 +882,7 @@ EVoxelConnectivityResult VF_AreConnected(
     TArray<int32> Path;
     if (!VoxelStrateMeasurePrivate::FindCoarsePath(Grid, Start, Goal, Path))
     {
-        return EVoxelConnectivityResult::NotConnected;
+        return EVoxelConnectivityResult::NotConnectedAtThisResolution;
     }
 
     // The coarse route is evidence, not a pass. Re-check this one BFS route at one-voxel spacing