feat(app): Phase A.2 — wire frustum culling into terrain + static-mesh renderers

Per-landblock AABB culling against the view frustum. Each loaded
landblock has a 192×192 XY footprint + a Z range derived from the
terrain vertex min/max (padded +50 above / -10 below for entities
on top and basements). One AABB test per landblock per frame;
landblocks fully outside the frustum skip ALL their terrain draws
and entity draws (both opaque and translucent passes).

GpuWorldState gains SetLandblockAabb + LandblockEntries (per-landblock
iteration with AABB data). TerrainRenderer.Draw and
StaticMeshRenderer.Draw both accept an optional FrustumPlanes and
skip culled landblocks. GameWindow.OnRender extracts FrustumPlanes
from camera.View * camera.Projection and passes to both.

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

src/AcDream.App/Streaming/GpuWorldState.cs

@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using System.Linq;
+using System.Numerics;
 using AcDream.Core.World;
 
 namespace AcDream.App.Streaming;
@@ -38,6 +39,7 @@ namespace AcDream.App.Streaming;
 public sealed class GpuWorldState
 {
     private readonly Dictionary<uint, LoadedLandblock> _loaded = new();
+    private readonly Dictionary<uint, (Vector3 Min, Vector3 Max)> _aabbs = new();
 
     /// <summary>
     /// Per-landblock buffer of live entities awaiting their landblock's
@@ -56,6 +58,34 @@ public sealed class GpuWorldState
 
     public bool IsLoaded(uint landblockId) => _loaded.ContainsKey(landblockId);
 
+    /// <summary>
+    /// Store the axis-aligned bounding box for a loaded landblock. Called from
+    /// the render thread after the terrain mesh is built and uploaded.
+    /// </summary>
+    public void SetLandblockAabb(uint landblockId, Vector3 min, Vector3 max)
+    {
+        _aabbs[landblockId] = (min, max);
+    }
+
+    /// <summary>
+    /// Per-landblock iteration with AABB data for use by the frustum-culling
+    /// draw path. Landblocks without a stored AABB yield <see cref="Vector3.Zero"/>
+    /// for both corners, which the culler will conservatively treat as visible.
+    /// </summary>
+    public IEnumerable<(uint LandblockId, Vector3 AabbMin, Vector3 AabbMax, IReadOnlyList<WorldEntity> Entities)> LandblockEntries
+    {
+        get
+        {
+            foreach (var kvp in _loaded)
+            {
+                if (_aabbs.TryGetValue(kvp.Key, out var aabb))
+                    yield return (kvp.Key, aabb.Min, aabb.Max, kvp.Value.Entities);
+                else
+                    yield return (kvp.Key, Vector3.Zero, Vector3.Zero, kvp.Value.Entities);
+            }
+        }
+    }
+
     /// <summary>
     /// Total live entities currently parked in the pending bucket waiting
     /// for their landblock to arrive. Useful diagnostic for verifying the
@@ -89,6 +119,7 @@ public sealed class GpuWorldState
         // pending spawns that arrived for it are no longer relevant. The
         // server will resend them via CreateObject when the player returns.
         _pendingByLandblock.Remove(landblockId);
+        _aabbs.Remove(landblockId);
 
         if (_loaded.Remove(landblockId))
             RebuildFlatView();