refactor(rendering): delete superseded visibility probes

Delete the callerless portal-BFS research graph and spent renderer probe families while retaining the production RetailFrameWalk path, terrain diagnostics, membership invariant, and walk transcript.

Mutation evidence (all restored):

1. Restored PortalVisibilityBuilder type -> AppAssembly_ContainsNoSupersededPortalGraphTypes first failed Assert.Empty with AcDream.App.Rendering.PortalVisibilityBuilder.

2. Restored ACDREAM_PROBE_FACILITY_STAIRS -> ProductionSource_ContainsNoDeletedRendererProbe_AndRetainsWalkTranscriptProof first failed Assert.Empty on RenderingDiagnostics.cs.

3. Added a second RetailFrameWalk field -> WalkFrameOwners_AreUnique first failed Assert.Single with _frameWalk and _mutatedSecondFrameWalk.

4. Added OrderBy to OrderedStream -> OrderedWalkStream_HasNoCrossStreamReorder first failed Assert.DoesNotContain on OrderBy(.

5. Added IDatReaderWriter parameter -> FrameTimeWalkOwners_HaveNoRawDatDependency first failed Assert.Empty on RetailFrameWalk.MutatedRawDatParameter.

tests/AcDream.App.Tests/Rendering/Walk/WalkFrameDriverTranscriptTests.cs

@@ -25,8 +25,8 @@ namespace AcDream.App.Tests.Rendering.Walk;
 /// <see cref="RenderingDiagnostics.DumpWalkTranscriptEnabled"/> and
 /// <see cref="Console.Out"/> are both process-wide mutable statics — joins
 /// <see cref="CameraDiagnosticsCollection"/> for the SAME reason
-/// <c>CornerFloodReplayTests</c>/<c>Issue181WallPressEquilibriumTests</c> do
-/// (that collection's own doc comment, issue #251): interleaved
+/// <c>Issue181WallPressEquilibriumTests</c> does (that collection's own doc
+/// comment, issue #251): interleaved
 /// <c>Console.SetOut</c> redirection across parallel test classes can
 /// restore a disposed <see cref="StringWriter"/> process-wide.
 /// </para>