test(A.5 T6): StreamingRegion transitions + hysteresis + oscillation coverage

Adds 5 tests to StreamingRegionTwoTierTests covering all tier-transition paths:
- FarToNear promote (walk 2 east from initial center)
- NullToNear teleport (loads 9 near + 40 far for a fully fresh region)
- NearToFar demote only after NearRadius+2 hysteresis threshold
- FarToNull unload only after FarRadius+2 hysteresis threshold
- oscillation no-thrash: bouncing 1 LB across a near boundary fires 0 demotes
  and at most 5 promotes total (one initial settle of the x=100 near-column)

Oscillation test fix: initialise the region at the oscillation midpoint
(103,100) rather than at a distant starting center (100,100) so the
initial move into the oscillation range doesn't itself trigger legitimate
demotes, isolating the no-thrash invariant.

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

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

@@ -53,4 +53,107 @@ public class StreamingRegionTwoTierTests
         }
         Assert.Empty(diff.ToLoadNear);
     }
+
+    [Fact]
+    public void RecenterTo_PlayerWalks_FarToNear_AppearsInToPromote()
+    {
+        var region = new StreamingRegion(centerX: 100, centerY: 100, nearRadius: 1, farRadius: 3);
+        _ = region.ComputeFirstTickDiff();
+        region.MarkResidentFromBootstrap();
+
+        // Walk 2 east — center (102, 100). LB (102, 100) was at distance 2 (Far)
+        // from (100,100); now at distance 0 → Near. That's a Promote.
+        var diff = region.RecenterTo(newCx: 102, newCy: 100);
+
+        var promotedId = StreamingRegion.EncodeLandblockIdForTest(102, 100);
+        Assert.Contains(promotedId, diff.ToPromote);
+        Assert.DoesNotContain(promotedId, diff.ToLoadNear);
+        Assert.DoesNotContain(promotedId, diff.ToLoadFar);
+    }
+
+    [Fact]
+    public void RecenterTo_PlayerTeleports_NullToNear_AppearsInToLoadNear()
+    {
+        var region = new StreamingRegion(centerX: 100, centerY: 100, nearRadius: 1, farRadius: 3);
+        _ = region.ComputeFirstTickDiff();
+        region.MarkResidentFromBootstrap();
+
+        // Teleport to (200, 200) — entirely new region.
+        var diff = region.RecenterTo(newCx: 200, newCy: 200);
+
+        Assert.Equal(9,  diff.ToLoadNear.Count);
+        Assert.Equal(40, diff.ToLoadFar.Count);
+        Assert.Empty(diff.ToPromote);
+    }
+
+    [Fact]
+    public void RecenterTo_NearToFar_DemoteOnlyAfterHysteresis()
+    {
+        // near=2, far=4 → near hysteresis threshold = 4.
+        var region = new StreamingRegion(centerX: 100, centerY: 100, nearRadius: 2, farRadius: 4);
+        _ = region.ComputeFirstTickDiff();
+        region.MarkResidentFromBootstrap();
+
+        // LB (100,100) was Near. Walk 3 east → distance 3 > NearRadius=2 but ≤ 4. No demote yet.
+        var diff1 = region.RecenterTo(newCx: 103, newCy: 100);
+        var lb100 = StreamingRegion.EncodeLandblockIdForTest(100, 100);
+        Assert.DoesNotContain(lb100, diff1.ToDemote);
+
+        // Walk 2 more east → distance 5 > 4. Demote.
+        var diff2 = region.RecenterTo(newCx: 105, newCy: 100);
+        Assert.Contains(lb100, diff2.ToDemote);
+    }
+
+    [Fact]
+    public void RecenterTo_FarToNull_UnloadOnlyAfterHysteresis()
+    {
+        var region = new StreamingRegion(centerX: 100, centerY: 100, nearRadius: 1, farRadius: 3);
+        _ = region.ComputeFirstTickDiff();
+        region.MarkResidentFromBootstrap();
+
+        // LB (97, 100) was at distance 3 (Far). Walk 1 east → distance 4. ≤ FarRadius+2=5.
+        var diff1 = region.RecenterTo(newCx: 101, newCy: 100);
+        var lb97 = StreamingRegion.EncodeLandblockIdForTest(97, 100);
+        Assert.DoesNotContain(lb97, diff1.ToUnload);
+
+        // Walk 2 more east → distance 6 > 5. Unload.
+        var diff2 = region.RecenterTo(newCx: 103, newCy: 100);
+        Assert.Contains(lb97, diff2.ToUnload);
+    }
+
+    [Fact]
+    public void RecenterTo_PlayerOscillates_NoThrashWithinHysteresis()
+    {
+        // Start the region centered on (103,100) so the oscillation
+        // between (102,100) and (103,100) never crosses a hysteresis boundary.
+        // NearRadius=2, farRadius=4 → nearUnloadThreshold=4.
+        // Chebyshev distance from (102,100) or (103,100) to any LB in the
+        // initial 9×9 window of (103,100) is ≤ NearRadius+2=4 for all LBs
+        // in the near zone, so no demotes should fire during pure oscillation.
+        var region = new StreamingRegion(centerX: 103, centerY: 100, nearRadius: 2, farRadius: 4);
+        _ = region.ComputeFirstTickDiff();
+        region.MarkResidentFromBootstrap();
+
+        // Bounce between (103,100) and (102,100). All resident LBs stay
+        // within the hysteresis window — no demotes expected.
+        int totalDemotes  = 0;
+        int totalPromotes = 0;
+        for (int i = 0; i < 5; i++)
+        {
+            var d1 = region.RecenterTo(102, 100);
+            totalDemotes  += d1.ToDemote.Count;
+            totalPromotes += d1.ToPromote.Count;
+            var d2 = region.RecenterTo(103, 100);
+            totalDemotes  += d2.ToDemote.Count;
+            totalPromotes += d2.ToPromote.Count;
+        }
+
+        // The first step from (103,100) to (102,100) legitimately promotes the
+        // x=100 near-column (5 LBs) that were Far from (103) into Near. After
+        // that initial settle they stay Near for all subsequent oscillations.
+        // So the ceiling is 5 promotes total (not per oscillation).
+        Assert.Equal(0, totalDemotes);
+        Assert.True(totalPromotes <= 5,
+            $"Expected ≤5 promotes across 5 oscillations; got {totalPromotes}");
+    }
 }