test(A.5 T3): StreamingRegion two-radius constructor

Add NearRadius/FarRadius properties and a four-arg constructor
(centerX, centerY, nearRadius, farRadius). Radius is set to farRadius
so existing hysteresis math (unload threshold = Radius+2) uses the
outer ring as the bookkeeping boundary. Old three-arg constructor
becomes a thin wrapper: this(cx, cy, radius, radius) — no behaviour
change, 25 pre-existing streaming tests still pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Streaming/StreamingRegion.cs

@@ -13,6 +13,8 @@ public sealed class StreamingRegion
     public int CenterX    { get; private set; }
     public int CenterY    { get; private set; }
     public int Radius     { get; }
+    public int NearRadius { get; }
+    public int FarRadius  { get; }
 
     // Strictly the (2r+1)×(2r+1) window (clamped to world bounds).
     private readonly HashSet<uint> _visible = new();
@@ -43,12 +45,16 @@ public sealed class StreamingRegion
     /// </summary>
     public IReadOnlyCollection<uint> Resident => _resident;
 
-    public StreamingRegion(int cx, int cy, int radius)
+    public StreamingRegion(int centerX, int centerY, int nearRadius, int farRadius)
     {
-        Radius = radius;
+        NearRadius = nearRadius;
-        Recenter(cx, cy);
+        FarRadius  = farRadius;
+        Radius     = farRadius;  // outer ring drives Resident bookkeeping
+        Recenter(centerX, centerY);
     }
 
+    public StreamingRegion(int cx, int cy, int radius) : this(cx, cy, radius, radius) { }
+
     private void Recenter(int cx, int cy)
     {
         CenterX = cx;

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

@@ -0,0 +1,18 @@
+using AcDream.App.Streaming;
+using Xunit;
+
+namespace AcDream.Core.Tests.Streaming;
+
+public class StreamingRegionTwoTierTests
+{
+    [Fact]
+    public void Constructor_TwoRadii_ExposesNearAndFarRadii()
+    {
+        var region = new StreamingRegion(centerX: 100, centerY: 100, nearRadius: 4, farRadius: 12);
+
+        Assert.Equal(4, region.NearRadius);
+        Assert.Equal(12, region.FarRadius);
+        Assert.Equal(100, region.CenterX);
+        Assert.Equal(100, region.CenterY);
+    }
+}