feat(app): Phase A.1 — StreamingController glue

Called once per frame from OnUpdate. Owns a StreamingRegion and uses
delegates into LandblockStreamer + a terrain-apply callback so unit
tests can inject fakes. Handles first-tick bootstrap (whole window
loads), boundary recenter (diff against previous center), and
drain completions (up to N per frame to cap GPU upload spikes).

4 new tests, all green.

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

tests/AcDream.Core.Tests/Streaming/StreamingControllerTests.cs

@@ -0,0 +1,106 @@
+using System.Collections.Generic;
+using AcDream.App.Streaming;
+using AcDream.Core.World;
+using DatReaderWriter.DBObjs;
+using Xunit;
+
+namespace AcDream.Core.Tests.Streaming;
+
+public class StreamingControllerTests
+{
+    private sealed class FakeStreamer
+    {
+        public List<uint> Loads { get; } = new();
+        public List<uint> Unloads { get; } = new();
+        public Queue<LandblockStreamResult> Pending { get; } = new();
+
+        public void EnqueueLoad(uint id) => Loads.Add(id);
+        public void EnqueueUnload(uint id) => Unloads.Add(id);
+        public IReadOnlyList<LandblockStreamResult> DrainCompletions(int max)
+        {
+            var batch = new List<LandblockStreamResult>();
+            while (batch.Count < max && Pending.Count > 0)
+                batch.Add(Pending.Dequeue());
+            return batch;
+        }
+    }
+
+    [Fact]
+    public void FirstTick_EnqueuesWholeVisibleWindow()
+    {
+        var state = new GpuWorldState();
+        var fake = new FakeStreamer();
+        var controller = new StreamingController(
+            enqueueLoad: fake.EnqueueLoad,
+            enqueueUnload: fake.EnqueueUnload,
+            drainCompletions: fake.DrainCompletions,
+            applyTerrain: _ => { },
+            state: state,
+            radius: 2);
+
+        // Center at (50, 50); no landblocks loaded yet.
+        controller.Tick(observerCx: 50, observerCy: 50);
+
+        // 5×5 window = 25 loads enqueued, 0 unloads.
+        Assert.Equal(25, fake.Loads.Count);
+        Assert.Empty(fake.Unloads);
+    }
+
+    [Fact]
+    public void SecondTick_SamePosition_EnqueuesNothing()
+    {
+        var state = new GpuWorldState();
+        var fake = new FakeStreamer();
+        var controller = new StreamingController(
+            fake.EnqueueLoad, fake.EnqueueUnload, fake.DrainCompletions,
+            _ => { }, state, radius: 2);
+
+        controller.Tick(50, 50);
+        fake.Loads.Clear();
+
+        controller.Tick(50, 50);
+
+        Assert.Empty(fake.Loads);
+        Assert.Empty(fake.Unloads);
+    }
+
+    [Fact]
+    public void DrainingLoadedResult_AddsToState()
+    {
+        var state = new GpuWorldState();
+        var fake = new FakeStreamer();
+        var applied = new List<LoadedLandblock>();
+        var controller = new StreamingController(
+            fake.EnqueueLoad, fake.EnqueueUnload, fake.DrainCompletions,
+            applied.Add, state, radius: 2);
+
+        // Note: LoadedLandblock's actual fields are LandblockId, Heightmap,
+        // Entities (positional record). Adjust if the first positional arg
+        // name differs.
+        var lb = new LoadedLandblock(0x32320FFEu, new LandBlock(), System.Array.Empty<WorldEntity>());
+        fake.Pending.Enqueue(new LandblockStreamResult.Loaded(0x32320FFEu, lb));
+
+        controller.Tick(50, 50);
+
+        Assert.Single(applied);
+        Assert.True(state.IsLoaded(0x32320FFEu));
+    }
+
+    [Fact]
+    public void DrainingUnloadedResult_RemovesFromState()
+    {
+        var state = new GpuWorldState();
+        var fake = new FakeStreamer();
+        var controller = new StreamingController(
+            fake.EnqueueLoad, fake.EnqueueUnload, fake.DrainCompletions,
+            _ => { }, state, radius: 2);
+
+        var lb = new LoadedLandblock(0x32320FFEu, new LandBlock(), System.Array.Empty<WorldEntity>());
+        state.AddLandblock(lb);
+        fake.Pending.Enqueue(new LandblockStreamResult.Unloaded(0x32320FFEu));
+
+        controller.Tick(50, 50);
+
+        Assert.False(state.IsLoaded(0x32320FFEu));
+    }
+}