fix(physics): don't fall back to no-collision resolve on transition fail

When FindTransitionalPosition fails (stuck in corner, too many steps),
use the partially-resolved position instead of falling back to the
simple Resolve which has no object collision. This prevents walking
through objects when the transition can't find a clean path.

The player now stops at corners instead of clipping through.

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

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -329,18 +329,26 @@ public sealed class PhysicsEngine
 
         bool ok = transition.FindTransitionalPosition(this);
 
+        var sp = transition.SpherePath;
+        var ci = transition.CollisionInfo;
+
         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.
+        // Transition failed (e.g., stuck in corner, too many steps).
-        return Resolve(currentPos, cellId, targetPos - currentPos, stepUpHeight);
+        // Use whatever position the transition reached (partial movement)
+        // instead of falling back to the no-collision Resolve.
+        // If CheckPos hasn't moved from CurPos, the player stays put —
+        // this is correct behavior when completely blocked.
+        bool partialOnGround = ci.ContactPlaneValid
+            || transition.ObjectInfo.State.HasFlag(ObjectInfoState.OnWalkable)
+            || isOnGround;
+
+        return new ResolveResult(sp.CheckPos, sp.CheckCellId != 0 ? sp.CheckCellId : cellId, partialOnGround);
     }
 }