feat(render): Phase A8 — RenderingDiagnostics.ProbeVisibilityEnabled

Adds the ACDREAM_PROBE_VIS=1 env-var-toggleable flag for the indoor-cell
visibility culling pipeline (#78). Mirrors the existing ProbeIndoor*
pattern. DebugVM checkbox follows.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Rendering/RenderingDiagnostics.cs

@@ -76,6 +76,16 @@ public static class RenderingDiagnostics
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_INDOOR_CULL") == "1"
         || Environment.GetEnvironmentVariable("ACDREAM_PROBE_INDOOR_ALL") == "1";
 
+    /// <summary>
+    /// Phase A8 (2026-05-25): when true, the indoor-cell stencil pipeline
+    /// emits one <c>[vis]</c> line per frame: camera-inside-cell flag,
+    /// VisibleCellIds count, HasExitPortalVisible flag, portal triangle
+    /// count uploaded, and which outdoor passes were stencil-gated.
+    /// Initial state from <c>ACDREAM_PROBE_VIS=1</c>.
+    /// </summary>
+    public static bool ProbeVisibilityEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_VIS") == "1";
+
     /// <summary>
     /// Master toggle. Reading reflects the AND of all five flags
     /// (true only when every probe is on). Writing cascades — setting

src/AcDream.UI.Abstractions/Panels/Debug/DebugVM.cs

@@ -345,6 +345,17 @@ public sealed class DebugVM
         set => RenderingDiagnostics.ProbeIndoorCullEnabled = value;
     }
 
+    /// <summary>
+    /// Phase A8 (2026-05-25). Runtime mirror of
+    /// <c>RenderingDiagnostics.ProbeVisibilityEnabled</c>
+    /// (env var <c>ACDREAM_PROBE_VIS</c>).
+    /// </summary>
+    public bool ProbeVisibility
+    {
+        get => RenderingDiagnostics.ProbeVisibilityEnabled;
+        set => RenderingDiagnostics.ProbeVisibilityEnabled = value;
+    }
+
     /// <summary>
     /// Indoor walking Phase 1 (2026-05-19). Runtime mirror of
     /// <c>PhysicsDiagnostics.ProbeIndoorBspEnabled</c> (env var

tests/AcDream.Core.Tests/Rendering/RenderingDiagnosticsVisibilityTests.cs

@@ -0,0 +1,27 @@
+// Phase A8 — visibility probe flag tests.
+
+using System;
+using AcDream.Core.Rendering;
+using Xunit;
+
+namespace AcDream.Core.Tests.Rendering;
+
+public class RenderingDiagnosticsVisibilityTests
+{
+    [Fact]
+    public void ProbeVisibilityEnabled_CanBeToggled()
+    {
+        var prev = RenderingDiagnostics.ProbeVisibilityEnabled;
+        try
+        {
+            RenderingDiagnostics.ProbeVisibilityEnabled = true;
+            Assert.True(RenderingDiagnostics.ProbeVisibilityEnabled);
+            RenderingDiagnostics.ProbeVisibilityEnabled = false;
+            Assert.False(RenderingDiagnostics.ProbeVisibilityEnabled);
+        }
+        finally
+        {
+            RenderingDiagnostics.ProbeVisibilityEnabled = prev;
+        }
+    }
+}