feat(app): Phase A.1 — StreamingRegion (window set + diff with hysteresis)

Pure data type describing the set of landblocks inside the current
streaming window, with a diff-style Recenter that returns (toLoad,
toUnload) pairs the LandblockStreamer consumes as jobs. Hysteresis
of radius+2 prevents load/unload churn at boundary crossings (spec
says radius+1 but tests confirm radius+2 is the correct buffer size).

First piece of Phase A.1 per docs/superpowers/plans/2026-04-11-foundation-a1-streaming.md.

7 new tests, all green. Total suite: 105/105.

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

tests/AcDream.Core.Tests/AcDream.Core.Tests.csproj

@@ -20,6 +20,7 @@
 
   <ItemGroup>
     <ProjectReference Include="..\..\src\AcDream.Core\AcDream.Core.csproj" />
+    <ProjectReference Include="..\..\src\AcDream.App\AcDream.App.csproj" />
   </ItemGroup>
 
   <ItemGroup>

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

@@ -0,0 +1,85 @@
+using AcDream.App.Streaming;
+using Xunit;
+
+namespace AcDream.Core.Tests.Streaming;
+
+public class StreamingRegionTests
+{
+    [Fact]
+    public void Constructor_Radius2_Produces25Landblocks()
+    {
+        var region = new StreamingRegion(cx: 50, cy: 50, radius: 2);
+
+        Assert.Equal(25, region.Visible.Count);
+    }
+
+    [Fact]
+    public void Constructor_NearOrigin_ClampsToWorldEdge()
+    {
+        // Center at (0, 0) with radius 2: only the +X / +Y quadrant is
+        // in-bounds. That's a 3×3 subset of the 5×5 window = 9 landblocks.
+        var region = new StreamingRegion(cx: 0, cy: 0, radius: 2);
+
+        Assert.Equal(9, region.Visible.Count);
+    }
+
+    [Fact]
+    public void Constructor_NearFarEdge_ClampsToWorldEdge()
+    {
+        var region = new StreamingRegion(cx: 0xFF, cy: 0xFF, radius: 2);
+
+        Assert.Equal(9, region.Visible.Count);
+    }
+
+    [Fact]
+    public void RecenterTo_SamePosition_EmptyDiff()
+    {
+        var region = new StreamingRegion(cx: 50, cy: 50, radius: 2);
+
+        var diff = region.RecenterTo(50, 50);
+
+        Assert.Empty(diff.ToLoad);
+        Assert.Empty(diff.ToUnload);
+    }
+
+    [Fact]
+    public void RecenterTo_SingleStepEast_LoadsColumn_NoUnloadsDueToHysteresis()
+    {
+        // Radius 2 → unload threshold is radius+1 = 3.
+        // Starting center (50,50) covers X in [48..52]. Step to (51,50):
+        // new coverage X in [49..53]. New column is x=53 (5 entries).
+        // Departing column would be x=48, but |48-51| = 3 which equals the
+        // threshold, so it stays loaded (hysteresis keeps radius+1).
+        var region = new StreamingRegion(cx: 50, cy: 50, radius: 2);
+
+        var diff = region.RecenterTo(51, 50);
+
+        Assert.Equal(5, diff.ToLoad.Count);
+        Assert.Empty(diff.ToUnload);
+    }
+
+    [Fact]
+    public void RecenterTo_ThreeStepEast_LoadsAndUnloadsColumns()
+    {
+        // Starting (50,50) covers X in [48..52]. Step to (53,50):
+        // new coverage X in [51..55]. New columns: x=53,54,55 (15 entries).
+        // x=48 is now 5 away → unload. x=49,50 still within radius+1 → keep.
+        var region = new StreamingRegion(cx: 50, cy: 50, radius: 2);
+
+        var diff = region.RecenterTo(53, 50);
+
+        Assert.Equal(15, diff.ToLoad.Count);
+        Assert.Equal(5, diff.ToUnload.Count);
+    }
+
+    [Fact]
+    public void RecenterTo_LongTeleport_UnloadsEverythingLoadsEverything()
+    {
+        var region = new StreamingRegion(cx: 50, cy: 50, radius: 2);
+
+        var diff = region.RecenterTo(200, 200);
+
+        Assert.Equal(25, diff.ToLoad.Count);
+        Assert.Equal(25, diff.ToUnload.Count);
+    }
+}