fix(core): Phase B.3 — add centroid + radius bounds to PortalPlane crossing test

Two targeted fixes for user-reported movement bugs:

1. Wall bounce: PortalPlane.FromVertices now accepts ALL polygon vertices
   (not just 3) for accurate centroid + bounding radius. IsCrossing uses
   2D (XY) distance check with tight radius (no multiplier) to prevent
   wall faces from triggering false indoor transitions. Walking along a
   building wall no longer launches the player into the air.

2. Slope alignment: PlayerMovementController adds a slope-proportional
   Z bias when walking uphill (up to +0.8 on steep slopes, grounded
   only). Prevents feet from sinking into the visual terrain mesh on
   slopes where the physics sample point lags the render surface.

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

src/AcDream.App/Input/PlayerMovementController.cs

@@ -88,6 +88,7 @@ public sealed class PlayerMovementController
 
     // Heartbeat timer.
     private float _heartbeatAccum;
+    private float _prevGroundZ;
     public const float HeartbeatInterval = 0.2f;  // 200ms
     public bool HeartbeatDue { get; private set; }
 
@@ -100,6 +101,7 @@ public sealed class PlayerMovementController
     {
         Position = pos;
         CellId = cellId;
+        _prevGroundZ = pos.Z;
     }
 
     public MovementResult Update(float dt, MovementInput input)
@@ -198,8 +200,19 @@ public sealed class PlayerMovementController
             }
         }
 
-        // Small upward bias prevents feet from z-fighting with terrain surface.
-        Position = new Vector3(result.Position.X, result.Position.Y, newZ + 0.15f);
+        // Upward bias prevents feet from sinking into the terrain surface.
+        // On slopes the visual terrain mesh rises ahead of the physics sample
+        // point, so we add extra bias proportional to how fast the ground Z is
+        // changing (steeper slope → more bias). Only apply when grounded — during
+        // jumps/falls the bias would interfere with the ballistic arc.
+        float slopeBias = 0f;
+        if (!IsAirborne)
+        {
+            float slopeDelta = MathF.Max(0f, newZ - _prevGroundZ);
+            slopeBias = MathF.Min(slopeDelta * 3f, 0.8f);
+        }
+        _prevGroundZ = newZ;
+        Position = new Vector3(result.Position.X, result.Position.Y, newZ + 0.15f + slopeBias);
         CellId = result.CellId;
 
         // 4. Determine current motion commands.