diag(phys): A6.P3 #98 — [step-walk-adjust] probe inside AdjustOffset

Adds one log line per AdjustOffset call (gated by ACDREAM_PROBE_STEP_WALK)
naming the branch taken (no-cp / no-cp-slide / slide-degenerate /
slide-crease / into-plane / away-plane, optionally +safety-push) plus
zGain = output.Z - input.Z.

No math or control-flow changes — pure observability so the next capture
can disambiguate the three failure-mode hypotheses for the cellar-ramp
climb cap. Re-reading the existing capture (a6-issue98-negpoly-...log)
showed the sphere DOES climb 90.00 -> 92.79 (2.79 m gain), then caps,
contradicting the divergence comparison's "no altitude gain" framing.
The real question is what stops the climb at world Z ~= 92.79 with the
cottage floor still 1.21 m higher. Existing [step-walk] probes wrap
AdjustOffset; this new probe reveals which branch the projection takes.

Fix plan with the four-branch decision tree at
docs/superpowers/plans/2026-05-23-a6-p3-issue98-cellar-up-fix.md.

Test baseline maintained: 1167 + 8.

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

src/AcDream.Core/Physics/PhysicsDiagnostics.cs

@@ -674,6 +674,49 @@ public static class PhysicsDiagnostics
             string.IsNullOrEmpty(detail) ? string.Empty : " " + detail));
     }
 
+    /// <summary>
+    /// A6.P3 issue #98 (2026-05-23) — focused probe INSIDE
+    /// <see cref="Transition.AdjustOffset"/> revealing which branch was
+    /// taken and the per-call Z gain. Pair with <c>[step-walk]
+    /// site=after-adjust</c> at the call site to triangulate where the
+    /// projection ends up. Caller MUST guard with
+    /// <c>if (!ProbeStepWalkEnabled) return;</c> before calling.
+    /// </summary>
+    public static void LogStepWalkAdjust(
+        string  branch,
+        Vector3 input,
+        Vector3 output,
+        Plane?  contactPlane,
+        bool    slidingValid,
+        Vector3 slidingNormal,
+        float   collisionAngle,
+        float   walkInterp)
+    {
+        var culture = System.Globalization.CultureInfo.InvariantCulture;
+
+        string cpDesc = contactPlane is { } cp
+            ? string.Format(culture,
+                "n=({0:F4},{1:F4},{2:F4}) d={3:F4}",
+                cp.Normal.X, cp.Normal.Y, cp.Normal.Z, cp.D)
+            : "n/a";
+
+        string slideDesc = slidingValid
+            ? string.Format(culture,
+                "({0:F4},{1:F4},{2:F4})",
+                slidingNormal.X, slidingNormal.Y, slidingNormal.Z)
+            : "n/a";
+
+        Console.WriteLine(string.Format(culture,
+            "[step-walk-adjust] branch={0} input=({1:F4},{2:F4},{3:F4}) " +
+            "output=({4:F4},{5:F4},{6:F4}) zGain={7:F4} " +
+            "cp={8} slide={9} colAngle={10:F4} winterp={11:F4}",
+            branch,
+            input.X, input.Y, input.Z,
+            output.X, output.Y, output.Z,
+            output.Z - input.Z,
+            cpDesc, slideDesc, collisionAngle, walkInterp));
+    }
+
     private static string FormatVector(bool valid, Vector3 value)
     {
         if (!valid)