feat(physics): ProbeWalkMissEnabled flag for ISSUES #83 H-disambiguation

Adds a new diagnostic flag for the indoor-walking walk-miss probe
spike per docs/superpowers/specs/2026-05-21-indoor-walk-miss-probe-design.md.
Env var ACDREAM_PROBE_WALK_MISS=1, runtime-toggleable via property.
No DebugPanel mirror — spike-only. Following commits wire the
[walk-miss] and [floor-polys] emissions to this flag.

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

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

@@ -0,0 +1,41 @@
+using AcDream.Core.Physics;
+using DatReaderWriter.Enums;
+using System.Collections.Generic;
+using System.Numerics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+/// <summary>
+/// Tests for the ISSUES #83 H-disambiguation probe spike (spec
+/// 2026-05-21-indoor-walk-miss-probe-design.md).
+///
+/// Covers:
+/// 1. PhysicsDiagnostics.ProbeWalkMissEnabled flag get/set roundtrip.
+/// 2. WalkMissDiagnostic.AggregateNearestWalkable selects the nearest
+///    walkable polygon by |dz| when the foot XY lies inside a poly's
+///    local XY bounding box.
+/// 3. WalkMissDiagnostic.AggregateNearestWalkable falls back to the
+///    nearest poly by |dz| when no walkable poly XY-contains the foot,
+///    reporting ContainsFootXY=false.
+/// </summary>
+public class WalkMissDiagnosticTests
+{
+    [Fact]
+    public void ProbeWalkMiss_StaticApi_Roundtrip()
+    {
+        bool initial = PhysicsDiagnostics.ProbeWalkMissEnabled;
+        try
+        {
+            PhysicsDiagnostics.ProbeWalkMissEnabled = true;
+            Assert.True(PhysicsDiagnostics.ProbeWalkMissEnabled);
+
+            PhysicsDiagnostics.ProbeWalkMissEnabled = false;
+            Assert.False(PhysicsDiagnostics.ProbeWalkMissEnabled);
+        }
+        finally
+        {
+            PhysicsDiagnostics.ProbeWalkMissEnabled = initial;
+        }
+    }
+}