feat(physics): A6.P1 — add ProbePushBackEnabled toggle

New PhysicsDiagnostics flag gates the [push-back] probe shipping
in subsequent tasks. Env-var ACDREAM_PROBE_PUSH_BACK=1 + DebugVM
mirror, matching the existing probe-toggle pattern.

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

src/AcDream.Core/Physics/PhysicsDiagnostics.cs

@@ -276,6 +276,34 @@ public static class PhysicsDiagnostics
     public static bool ProbeWalkMissEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_WALK_MISS") == "1";
 
+    /// <summary>
+    /// Phase A6.P1 cdb probe spike (2026-05-21). When true, every BSP
+    /// collision response site emits a structured <c>[push-back]</c> line:
+    /// input/output sphere center, plane geometry, push-back delta, walk
+    /// interp, and the dispatcher's selected path. Direct comparison to
+    /// retail's cdb breakpoint set documented at
+    /// <c>tools/cdb/a6-probe.cdb</c>.
+    ///
+    /// <para>
+    /// Three emission sites: <see cref="BSPQuery.AdjustSphereToPlane"/>
+    /// (the suspected over-correction site), <see cref="BSPQuery.FindCollisions"/>
+    /// (the 6-path dispatcher), and <see cref="Transition.CheckOtherCells"/>
+    /// (multi-cell BSP iteration outcomes). All three are zero-cost when
+    /// off — checked via early-out at each site.
+    /// </para>
+    ///
+    /// <para>
+    /// Initial state from <c>ACDREAM_PROBE_PUSH_BACK=1</c>.
+    /// Runtime-toggleable via DebugVM mirror.
+    /// </para>
+    ///
+    /// <para>
+    /// Spec: <c>docs/superpowers/specs/2026-05-21-phase-a6-indoor-physics-fidelity-design.md</c>.
+    /// </para>
+    /// </summary>
+    public static bool ProbePushBackEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_PUSH_BACK") == "1";
+
     public static void LogCpBoolWrite(string field, bool oldValue, bool newValue)
     {
         var caller = GetCpCallerName();

tests/AcDream.Core.Tests/Physics/PhysicsDiagnosticsTests.cs

@@ -95,4 +95,27 @@ public class PhysicsDiagnosticsTests
             PhysicsDiagnostics.LastBspHitPoly = initial;
         }
     }
+
+    // -----------------------------------------------------------------------
+    // ProbePushBackEnabled — flag gates the [push-back] emission path.
+    // A6.P1 (2026-05-21).
+    // -----------------------------------------------------------------------
+
+    [Fact]
+    public void ProbePushBack_StaticApi_Roundtrip()
+    {
+        bool initial = PhysicsDiagnostics.ProbePushBackEnabled;
+        try
+        {
+            PhysicsDiagnostics.ProbePushBackEnabled = true;
+            Assert.True(PhysicsDiagnostics.ProbePushBackEnabled);
+
+            PhysicsDiagnostics.ProbePushBackEnabled = false;
+            Assert.False(PhysicsDiagnostics.ProbePushBackEnabled);
+        }
+        finally
+        {
+            PhysicsDiagnostics.ProbePushBackEnabled = initial;
+        }
+    }
 }