fix(app): multi-point Z sampling + never-cull player landblock

1. Slope clipping: replaced single foot-forward Z sample with 4-point
   sampling (forward, back, left, right at 0.7 units). Takes the max Z
   across all samples so both uphill and downhill slopes keep feet above
   the terrain mesh surface. Removed the +0.1 Z bias entirely.

2. Player culling: replaced per-entity scan (alwaysVisibleEntityId) with
   per-landblock skip (neverCullLandblockId). The player's current
   landblock is computed from _playerController.Position and passed to
   the renderer. Simpler, faster, and more reliable.

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

src/AcDream.App/Input/PlayerMovementController.cs

@@ -206,22 +206,32 @@ public sealed class PlayerMovementController
             }
         }
 
-        // Foot-forward Z sampling: on slopes the visual terrain at the
-        // character's feet (slightly forward) can be higher than at the center.
-        // Sample a point ~1 unit ahead in the walk direction and use the MAX
-        // of center and foot Z. Only when grounded and moving.
-        if (!IsAirborne && (dx != 0f || dy != 0f))
+        // Multi-point Z sampling: on slopes the visual terrain mesh can be
+        // higher or lower than the center-point physics sample. Sample 4
+        // points around the character (forward, back, left, right at ~0.7
+        // units — roughly foot distance) and use the MAX Z. This prevents
+        // feet clipping on both uphill and downhill slopes.
+        if (!IsAirborne)
         {
-            float footX = result.Position.X + forwardX * 1.0f;
-            float footY = result.Position.Y + forwardY * 1.0f;
-            var footResult = _physics.Resolve(
-                new Vector3(footX, footY, newZ), CellId,
-                Vector3.Zero, StepUpHeight);
-            if (footResult.IsOnGround && footResult.Position.Z > newZ)
-                newZ = footResult.Position.Z;
+            const float sampleDist = 0.7f;
+            ReadOnlySpan<(float ox, float oy)> offsets = stackalloc (float, float)[]
+            {
+                ( forwardX * sampleDist,  forwardY * sampleDist),  // forward
+                (-forwardX * sampleDist, -forwardY * sampleDist),  // back
+                ( forwardY * sampleDist, -forwardX * sampleDist),  // right
+                (-forwardY * sampleDist,  forwardX * sampleDist),  // left
+            };
+            foreach (var (ox, oy) in offsets)
+            {
+                var sampleResult = _physics.Resolve(
+                    new Vector3(result.Position.X + ox, result.Position.Y + oy, newZ),
+                    CellId, Vector3.Zero, StepUpHeight);
+                if (sampleResult.IsOnGround && sampleResult.Position.Z > newZ)
+                    newZ = sampleResult.Position.Z;
+            }
         }
         _prevGroundZ = newZ;
-        Position = new Vector3(result.Position.X, result.Position.Y, newZ + 0.1f);
+        Position = new Vector3(result.Position.X, result.Position.Y, newZ);
         CellId = result.CellId;
 
         // 4. Determine current motion commands.

src/AcDream.App/Rendering/GameWindow.cs

@@ -1757,8 +1757,17 @@ public sealed class GameWindow : IDisposable
             var camera = _cameraController.Active;
             var frustum = AcDream.App.Rendering.FrustumPlanes.FromViewProjection(camera.View * camera.Projection);
             _terrain?.Draw(camera, frustum);
+            // Never cull the landblock the player is currently on.
+            uint? playerLb = null;
+            if (_playerMode && _playerController is not null)
+            {
+                var pp = _playerController.Position;
+                int plx = _liveCenterX + (int)System.Math.Floor(pp.X / 192f);
+                int ply = _liveCenterY + (int)System.Math.Floor(pp.Y / 192f);
+                playerLb = (uint)((plx << 24) | (ply << 16) | 0xFFFF);
+            }
             _staticMesh?.Draw(camera, _worldState.LandblockEntries, frustum,
-                alwaysVisibleEntityId: _playerMode ? _playerServerGuid : null);
+                neverCullLandblockId: playerLb);
 
             // Count visible vs total for the perf overlay.
             foreach (var entry in _worldState.LandblockEntries)

src/AcDream.App/Rendering/StaticMeshRenderer.cs

@@ -79,7 +79,7 @@ public sealed unsafe class StaticMeshRenderer : IDisposable
     public void Draw(ICamera camera,
         IEnumerable<(uint LandblockId, Vector3 AabbMin, Vector3 AabbMax, IReadOnlyList<WorldEntity> Entities)> landblockEntries,
         FrustumPlanes? frustum = null,
-        uint? alwaysVisibleEntityId = null)
+        uint? neverCullLandblockId = null)
     {
         _shader.Use();
         _shader.SetMatrix4("uView", camera.View);
@@ -90,21 +90,11 @@ public sealed unsafe class StaticMeshRenderer : IDisposable
         // alpha-discard path in the fragment shader (uTranslucencyKind == 1).
         foreach (var entry in landblockEntries)
         {
-            // Per-landblock frustum cull: one AABB test skips all entities in
-            // this landblock if it is fully outside the view frustum.
-            // Exception: never cull a landblock containing the player entity.
+            // Per-landblock frustum cull. Never cull the player's landblock.
             if (frustum is not null &&
+                entry.LandblockId != neverCullLandblockId &&
                 !FrustumCuller.IsAabbVisible(frustum.Value, entry.AabbMin, entry.AabbMax))
-            {
-                // Check if this landblock contains the always-visible entity.
-                bool hasAlwaysVisible = false;
-                if (alwaysVisibleEntityId is not null)
-                {
-                    foreach (var e in entry.Entities)
-                        if (e.Id == alwaysVisibleEntityId.Value) { hasAlwaysVisible = true; break; }
-                }
-                if (!hasAlwaysVisible) continue;
-            }
+                continue;
 
             foreach (var entity in entry.Entities)
             {
@@ -179,18 +169,11 @@ public sealed unsafe class StaticMeshRenderer : IDisposable
 
         foreach (var entry in landblockEntries)
         {
-            // Same per-landblock frustum cull + always-visible exception for pass 2.
+            // Same per-landblock frustum cull for pass 2.
             if (frustum is not null &&
+                entry.LandblockId != neverCullLandblockId &&
                 !FrustumCuller.IsAabbVisible(frustum.Value, entry.AabbMin, entry.AabbMax))
-            {
-                bool hasAlwaysVisible = false;
-                if (alwaysVisibleEntityId is not null)
-                {
-                    foreach (var e in entry.Entities)
-                        if (e.Id == alwaysVisibleEntityId.Value) { hasAlwaysVisible = true; break; }
-                }
-                if (!hasAlwaysVisible) continue;
-            }
+                continue;
 
             foreach (var entity in entry.Entities)
             {

tests/AcDream.Core.Tests/Input/PlayerMovementControllerTests.cs

@@ -137,7 +137,7 @@ public class PlayerMovementControllerTests
 
         Assert.False(controller.IsAirborne, "Should have landed");
         // +0.15 Z bias keeps feet above terrain surface (prevents z-fighting).
-        Assert.Equal(50.1f, controller.Position.Z, precision: 1);
+        Assert.Equal(50f, controller.Position.Z, precision: 1);
     }
 
     [Fact]
@@ -177,6 +177,6 @@ public class PlayerMovementControllerTests
             controller.Update(0.05f, new MovementInput(Forward: true));
 
         Assert.False(controller.IsAirborne, "Player should have landed");
-        Assert.Equal(20.1f, controller.Position.Z, precision: 1);
+        Assert.Equal(20f, controller.Position.Z, precision: 1);
     }
 }