fix(items): use retail cylinder range for loot windows

Keep external-container visibility tied to the signed surface gap between the player and container physics cylinders, matching gmExternalContainerUI's authored UseRadius range watcher. This prevents a valid ViewContents response from being closed at ACE's natural approach endpoint.

Port Position::cylinder_distance and ACCWeenieObject::ObjectsInRange into Core with conformance coverage for 3-D separation, overlap, mode precedence, and inclusive boundaries.

Release build succeeds and all 5,895 tests pass with five intentional skips.

Co-authored-by: OpenAI Codex <codex@openai.com>

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

@@ -0,0 +1,68 @@
+using System.Numerics;
+using AcDream.Core.Physics;
+
+namespace AcDream.Core.Tests.Physics;
+
+/// <summary>
+/// Conformance coverage for retail
+/// <c>ACCWeenieObject::ObjectsInRange @ 0x0058C1A0</c>. The external-container
+/// registration passes <c>useRadii=true</c> and <c>ignoreZDelta=false</c> at
+/// <c>gmExternalContainerUI::SetGroundObject @ 0x004CBBD0</c>.
+/// </summary>
+public sealed class ObjectRangeMathTests
+{
+    [Fact]
+    public void ExternalContainerRange_UsesSurfaceGapRatherThanCenterDistance()
+    {
+        var player = new Vector3(7.01f, 98.06f, -0.45f);
+        var corpse = new Vector3(3.73f, 98.03f, -0.44f);
+
+        bool inRange = ObjectRangeMath.ObjectsInRange(
+            player, firstRadius: 0.48f, firstHeight: 1.835f,
+            corpse, secondRadius: 0.80f, secondHeight: 0.50f,
+            range: 2.01,
+            useRadii: true,
+            ignoreZDelta: false);
+
+        Assert.True(inRange);
+    }
+
+    [Fact]
+    public void CenterDistanceMode_DoesNotSubtractObjectRadii()
+    {
+        bool inRange = ObjectRangeMath.ObjectsInRange(
+            Vector3.Zero, firstRadius: 10f, firstHeight: 10f,
+            new Vector3(3f, 0f, 0f), secondRadius: 10f, secondHeight: 10f,
+            range: 2.0,
+            useRadii: false,
+            ignoreZDelta: false);
+
+        Assert.False(inRange);
+    }
+
+    [Fact]
+    public void IgnoreZDelta_UsesXyCenterDistanceAndTakesPrecedenceOverRadii()
+    {
+        bool inRange = ObjectRangeMath.ObjectsInRange(
+            Vector3.Zero, firstRadius: 100f, firstHeight: 100f,
+            new Vector3(3f, 4f, 1000f), secondRadius: 100f, secondHeight: 100f,
+            range: 5.0,
+            useRadii: true,
+            ignoreZDelta: true);
+
+        Assert.True(inRange);
+    }
+
+    [Fact]
+    public void RangeBoundary_IsInclusive()
+    {
+        bool inRange = ObjectRangeMath.ObjectsInRange(
+            Vector3.Zero, firstRadius: 0f, firstHeight: 0f,
+            new Vector3(3f, 4f, 0f), secondRadius: 0f, secondHeight: 0f,
+            range: 5.0,
+            useRadii: false,
+            ignoreZDelta: false);
+
+        Assert.True(inRange);
+    }
+}