feat(render-diag): add ACDREAM_PROBE_PORTAL_CHURN flag for the bounded-propagation pin

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Rendering/RenderingDiagnostics.cs

@@ -143,6 +143,17 @@ public static class RenderingDiagnostics
     public static bool ProbePvInputEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_PVINPUT") == "1";
 
+    /// <summary>
+    /// Bounded-propagation port apparatus (2026-06-08). When true, PortalVisibilityBuilder.Build emits
+    /// one [portal-churn] summary line per call: per-cell pop count (re-pops = churn), total re-enqueues,
+    /// max pop count, and — per re-enqueue — the reciprocal-clip pre→post region count + grew flag. Pins
+    /// whether the flap's churn is redundant reciprocal back-contributions producing non-empty drifted
+    /// slivers (the hypothesis) vs another source. Throwaway apparatus — strip once the bound ships.
+    /// Initial state from ACDREAM_PROBE_PORTAL_CHURN=1.
+    /// </summary>
+    public static bool ProbePortalChurnEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_PORTAL_CHURN") == "1";
+
     // Cell-change gate for EmitVis. The probe fires once per distinct root cell
     // so launch.log stays readable under motion (the per-frame call is a no-op
     // when the root is unchanged). Sentinel 0 = "no root yet" — the first real

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

@@ -132,4 +132,11 @@ public sealed class RenderingDiagnosticsTests
         // playerCellId == 0 (unresolved) → treat as outside (safe default: outdoor render).
         Assert.False(RenderingDiagnostics.ShouldRenderIndoor(playerCellId: 0u, renderRootResolved: true));
     }
+
+    [Fact]
+    public void ProbePortalChurn_DefaultsFalse_WhenEnvUnset()
+    {
+        // Env var is absent in the test host, so the flag must default false (inert probe).
+        Assert.False(AcDream.Core.Rendering.RenderingDiagnostics.ProbePortalChurnEnabled);
+    }
 }