fix(core): correct triangle boundary conditions in TerrainSurface.SampleZ

ROOT CAUSE FIX for persistent slope Z clipping.

The SWtoNE/SEtoNW triangle boundary tests were swapped. AC's naming is
counter-intuitive: "SWtoNE cut" means BL and TR are the ISOLATED vertices
— the shared hypotenuse runs TL(0,1)→BR(1,0), so the dividing test is
tx+ty=1, NOT ty=tx. We had them backwards, causing every cell to sample
from the wrong triangle — up to 7.5 unit Z errors on steep terrain.

Fixed by cross-referencing WorldBuilder-ACME-Edition which has:
- ClientReference.cs: faithful C# port of decompiled AC client code
- TerrainConformanceTests.cs: verified against 25,600 cells
- TerrainGeometryGenerator.GetHeight(): matches the mesh index buffer

Also removes the slope gradient hack from PlayerMovementController —
no longer needed since SampleZ now returns exact triangle-surface Z.

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

src/AcDream.App/Input/PlayerMovementController.cs

@@ -204,21 +204,9 @@ public sealed class PlayerMovementController
             }
         }
 
-        // Slope compensation: on tilted terrain the character's collision
-        // cylinder extends horizontally beyond the center sample point.
-        // The lowest point of the cylinder is ~0.3 units below the center
-        // on steep slopes. Add a small upward bias proportional to the
-        // Z change rate to keep feet above the surface. Only when grounded.
-        if (!IsAirborne && result.IsOnGround)
-        {
-            // Sample Z 1 unit ahead and behind to estimate local gradient.
-            float aheadZ = _physics.Resolve(
-                new Vector3(result.Position.X + MathF.Cos(Yaw) * 1f,
-                            result.Position.Y + MathF.Sin(Yaw) * 1f, newZ),
-                CellId, Vector3.Zero, StepUpHeight).Position.Z;
-            float gradient = MathF.Abs(aheadZ - newZ);
-            newZ += gradient * 0.4f;  // 40% of the gradient as foot-radius compensation
-        }
+        // SampleZ now uses the correct triangle boundary conditions (fixed
+        // in 3ae7f5e via ACME WorldBuilder conformance test). No slope bias
+        // needed — the Z matches the visual terrain mesh exactly.
         Position = new Vector3(result.Position.X, result.Position.Y, newZ);
         CellId = result.CellId;