refactor(rendering): delete superseded visibility probes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/EnvCellAlphaDrawSourceTests.cs

@@ -738,7 +738,6 @@ public sealed class EnvCellAlphaDrawSourceTests
                 particleRenderer: null,
                 portalDepthMask: null,
                 Queue,
-                (WorldRenderDiagnostics)RuntimeHelpers.GetUninitializedObject(typeof(WorldRenderDiagnostics)),
                 (TerrainDrawDiagnosticsController)RuntimeHelpers.GetUninitializedObject(
                     typeof(TerrainDrawDiagnosticsController)));
             Leaf = new WalkProductionLeafRenderer(