docs(physics): #347 premise revision — retail may alternate too; ftp:edge ratio is the discriminator

The cliff_slide arms are conformant in ACE, our port, and the bytes
(compare constant at 0x794610 verified 0.0), the round-1 slidn:edge
ratio (538:594) refutes a retail retry storm, and the user's
side-by-side speed observation fits alternation. Round-2 cdb script
now counts find_transitional_position; H-A (identical, retire AD-70)
vs H-B (within-tick yield) resolves on one ratio. The temporary
Scratch347 diagnostic test rides along until #347 closes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/AcDream.Core.Tests/Physics/Scratch347DiagTests.cs

@@ -0,0 +1,72 @@
+using System;
+using System.IO;
+using System.Numerics;
+using AcDream.Core.Physics;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace AcDream.Core.Tests.Physics;
+
+// TEMPORARY #347 diagnostic — deleted before landing.
+public sealed class Scratch347DiagTests
+{
+    private readonly ITestOutputHelper _out;
+    public Scratch347DiagTests(ITestOutputHelper output) => _out = output;
+
+    [Fact]
+    public void DumpFirstArmingTick()
+    {
+        var heights = new byte[81];
+        heights[3 * 9 + 4] = 32;
+        var heightTable = new float[256];
+        for (int i = 0; i < 256; i++) heightTable[i] = i;
+
+        var engine = new PhysicsEngine();
+        engine.AddLandblock(0xA9B4FFFFu, new TerrainSurface(heights, heightTable),
+            Array.Empty<CellSurface>(), Array.Empty<PortalPlane>(), 0f, 0f);
+
+        var body = new PhysicsBody
+        {
+            State = PhysicsStateFlags.Gravity,
+            TransientState = TransientStateFlags.Active | TransientStateFlags.Contact | TransientStateFlags.OnWalkable,
+        };
+
+        float dx = 0f, dy = 0.23f;
+        var position = new Vector3(80.4f, 79.8f, 0f);
+        uint cell = TerrainSurface.ComputeOutdoorCellId(0xA9B4FFFFu, 80.4f, 79.8f);
+
+        PhysicsDiagnostics.DumpTransitFailEnabled = true;
+        var saved = Console.Out;
+        var sw = new StringWriter();
+        Console.SetOut(sw);
+        try
+        {
+            for (int tick = 0; tick < 3; tick++)
+            {
+                var result = engine.ResolveWithTransition(
+                    currentPos: position,
+                    targetPos: new Vector3(position.X + dx, position.Y + dy, position.Z),
+                    cellId: cell,
+                    sphereRadius: 0.47f,
+                    sphereHeight: 1.20f,
+                    stepUpHeight: 0.60f,
+                    stepDownHeight: 1.50f,
+                    isOnGround: true,
+                    body: body,
+                    moverFlags: ObjectInfoState.IsPlayer | ObjectInfoState.EdgeSlide,
+                    movingEntityId: 0x5000000Au);
+                position = result.Position;
+                cell = result.CellId;
+                if (tick < 2) sw.GetStringBuilder().Clear();  // keep only the arming tick
+            }
+        }
+        finally
+        {
+            Console.SetOut(saved);
+            PhysicsDiagnostics.DumpTransitFailEnabled = false;
+        }
+
+        foreach (var line in sw.ToString().Split('\n'))
+            _out.WriteLine(line.TrimEnd());
+    }
+}