feat(physics): use sloped terrain plane in FindEnvCollisions

Our previous FindEnvCollisions built a FLAT contact plane (Normal = +Z)
at the sampled terrain Z, discarding the triangle's actual slope.
Retail uses the real terrain polygon's plane (ACE Landblock.cs:125-137
find_terrain_poly → walkable.Plane) which IS sloped.

Without a true slope normal, AdjustOffset's projection of horizontal
velocity onto the plane produces no slope-aligned Z component — fine
for step-subdivision on flat ground, visibly wrong whenever the contact
plane is carried across frames (via PhysicsBody.ContactPlane persistence
from commit 93cbabb): the projection is a no-op and movement is purely
kinematic. With the real slope normal, projected motion correctly
follows the slope.

Not a user-visible bug fix by itself (DIAG LocalZ shows delta≈0 for the
local player everywhere; the "looks too high in water" issue the user
reported is actually a missing water-rendering feature, not a physics
bug). Landing it anyway because it matches retail behavior and removes
the "flat-plane-is-fine" assumption that would bite on any future
contact-plane-dependent code.

Additions:
 - TerrainSurface.SampleSurface(localX, localY) → (Z, Normal), deriving
   the plane normal analytically from the triangle's height gradient.
   Matches the same triangle SampleZ already interpolates through.
 - PhysicsEngine.SampleTerrainPlane(worldX, worldY) → System.Numerics.Plane,
   the wrapper that bridges terrain space to transition space.
 - TransitionTypes.FindEnvCollisions uses SampleTerrainPlane instead of
   synthesizing a flat plane from SampleTerrainZ.

All 717 tests green. Flat-plane case is unchanged (Normal.Z = 1 when
the triangle is level, identical to the old plane).

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

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -110,6 +110,35 @@ public sealed class PhysicsEngine
         return null;
     }
 
+    /// <summary>
+    /// Sample the outdoor terrain plane (Z + sloped normal) at the given
+    /// world-space XY position. The returned <see cref="System.Numerics.Plane"/>
+    /// has the true terrain-triangle normal (NOT a flat <c>(0,0,1)</c>), and
+    /// its <c>D</c> is set so the plane passes through the sampled point. Used
+    /// by <see cref="Transition"/> to build a CORRECT contact plane — a flat
+    /// plane breaks slope tracking because <c>AdjustOffset</c>'s projection
+    /// onto a flat plane cannot impart the Z component that horizontal
+    /// velocity needs to follow the slope.
+    /// </summary>
+    public System.Numerics.Plane? SampleTerrainPlane(float worldX, float worldY)
+    {
+        foreach (var kvp in _landblocks)
+        {
+            var lb = kvp.Value;
+            float localX = worldX - lb.WorldOffsetX;
+            float localY = worldY - lb.WorldOffsetY;
+            if (localX >= 0f && localX < 192f && localY >= 0f && localY < 192f)
+            {
+                var (z, normal) = lb.Terrain.SampleSurface(localX, localY);
+                // System.Numerics.Plane convention: dot(Normal, P) + D == 0
+                // for points P on the plane. Pick P = (worldX, worldY, z).
+                float d = -(normal.X * worldX + normal.Y * worldY + normal.Z * z);
+                return new System.Numerics.Plane(normal, d);
+            }
+        }
+        return null;
+    }
+
     /// <summary>
     /// Resolve an entity's movement from <paramref name="currentPos"/> by
     /// applying <paramref name="delta"/> (XY only) and computing the correct Z