fix(physics): L.4-cliffslide-priority — steep ContactPlane check before OnWalkable gate

User-reported: "still don't slide down steep roofs" after the previous
trigger-gate fix (52e257d). Traced through the EdgeSlide dispatcher:
the gate IS firing now, but ValidateTransition's L.2.3i FloorZ test
clears OnWalkable as soon as the player is on a steep surface. So
EdgeSlideAfterStepDownFailed enters Branch 1 (`!OnWalkable → restore
+ OK`) and stops the player BEFORE Branch 2's steep-ContactPlane
CliffSlide can fire.

Re-order: check the steep-ContactPlane condition FIRST, before the
Branch 1 OnWalkable gate. If the surface is too steep AND we have a
contact plane on it AND the EdgeSlide flag is set, run CliffSlide
regardless of OnWalkable state. The cross-product deflection plus
gravity produces continuous downhill drift, frame after frame.

Branch 1's "stop at edge" still fires for the original case it was
meant for: walked off into thin air with no contact plane at all.
That should still stop (or fall normally) rather than CliffSlide
against nothing.

Tests: 1491 still pass.

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

src/AcDream.Core/Physics/TransitionTypes.cs

@@ -810,6 +810,35 @@ public sealed class Transition
         var ci = CollisionInfo;
         var oi = ObjectInfo;
 
+        // L.4-cliffslide-priority (2026-04-30): the steep-ContactPlane check
+        // moved BEFORE the OnWalkable/EdgeSlide gate.
+        //
+        // Why: by the time this dispatch runs on subsequent frames (player
+        // standing on a steep slope), ValidateTransition's L.2.3i FloorZ
+        // test has already CLEARED OnWalkable (steep slope → not a walkable
+        // surface). The original Branch 1 (`!OnWalkable → restore + OK`)
+        // therefore fires every frame, stopping the player dead — exactly
+        // the "stay on the roof" symptom the user reported.
+        //
+        // Re-ordering: if the surface is too steep AND we have a contact
+        // plane on it, run CliffSlide regardless of OnWalkable. The
+        // cross(currentNormal, lastKnownNormal) deflection plus gravity
+        // produces visible downhill drift each frame.
+        //
+        // Branch 1 (the !OnWalkable stop) still fires when we DON'T have
+        // a contact plane — the original "walked off into thin air"
+        // case, which should still stop or fall normally rather than
+        // CliffSlide on nothing.
+        if (ci.ContactPlaneValid && ci.ContactPlane.Normal.Z < zVal && oi.EdgeSlide)
+        {
+            var cliffPlane = ci.ContactPlane;
+            sp.ClearWalkable();
+            sp.RestoreCheckPos();
+            ci.ContactPlaneValid = false;
+            ci.ContactPlaneIsWater = false;
+            return CliffSlide(cliffPlane);
+        }
+
         // Retail lets non-EdgeSlide movers continue over the boundary. Player
         // movement carries EdgeSlide, so the local avatar takes the slide path.
         if (!oi.OnWalkable || !oi.EdgeSlide)