feat(physics): wire CTransition sphere-sweep into player movement

Replace simple Z-snap PhysicsEngine.Resolve with ResolveWithTransition
that uses the ported CTransition sphere-sweep pipeline. Movement is
subdivided into sphere-radius steps, terrain collision tested at each
step with step-down for ground contact maintenance.

Falls back to simple Resolve if transition fails. Player controller
now passes pre/post integration positions to the transition system.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -259,4 +259,43 @@ public sealed class PhysicsEngine
             targetCellId,
             IsOnGround: true);
     }
+
+    /// <summary>
+    /// Resolve movement using the CTransition sphere-sweep system.
+    /// Subdivides movement into sphere-radius steps, tests terrain collision
+    /// at each step, handles step-down for ground contact.
+    /// Falls back to the simple <see cref="Resolve"/> if the transition fails.
+    /// </summary>
+    public ResolveResult ResolveWithTransition(
+        Vector3 currentPos, Vector3 targetPos, uint cellId,
+        float sphereRadius, float sphereHeight,
+        float stepUpHeight, float stepDownHeight,
+        bool isOnGround)
+    {
+        var transition = new Transition();
+        transition.ObjectInfo.StepUpHeight = stepUpHeight;
+        transition.ObjectInfo.StepDownHeight = stepDownHeight;
+        transition.ObjectInfo.StepDown = true;
+
+        if (isOnGround)
+            transition.ObjectInfo.State |= ObjectInfoState.Contact | ObjectInfoState.OnWalkable;
+
+        transition.SpherePath.InitPath(currentPos, targetPos, cellId, sphereRadius, sphereHeight);
+
+        bool ok = transition.FindTransitionalPosition(this);
+
+        if (ok)
+        {
+            var sp = transition.SpherePath;
+            var ci = transition.CollisionInfo;
+
+            bool onGround = ci.ContactPlaneValid
+                || transition.ObjectInfo.State.HasFlag(ObjectInfoState.OnWalkable);
+
+            return new ResolveResult(sp.CheckPos, sp.CheckCellId, onGround);
+        }
+
+        // Transition failed — fall back to simple resolve.
+        return Resolve(currentPos, cellId, targetPos - currentPos, stepUpHeight);
+    }
 }