fix(G.3): hysteresis on the dungeon streaming gate — stop collapse↔expand thrash (#133)

The first cut of the dungeon gate keyed expand on the per-frame insideDungeon
signal (CurrCell is a sealed EnvCell). Live, CurrCell momentarily resolves to
null mid-frame while the player stays put in the dungeon landblock, so the gate
flipped collapse→expand→collapse every few frames. Each expand re-streamed the
full 25×25 window; the unloads couldn't keep up (MaxCompletionsPerFrame=4), so
registered lights leaked to 212k and FPS spiked to single digits between the
~199 fps collapsed frames.

Fix: once collapsed, key the gate on the STABLE observer landblock, not CurrCell.
Stay collapsed while the player remains in the dungeon landblock (_collapsedCenter);
expand only when the observer actually moves to a different landblock (portal/
teleport out). CurrCell flicker no longer thrashes.

Regression test added (Collapsed_CurrCellFlickersToNull_SameLandblock_DoesNotExpand).
Build green; 60 streaming tests green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -109,6 +109,25 @@ public class StreamingControllerDungeonGateTests
         Assert.Empty(h.Loads);                    // no spurious center reloads
     }
 
+    [Fact]
+    public void Collapsed_CurrCellFlickersToNull_SameLandblock_DoesNotExpand()
+    {
+        // Regression: the live run thrashed collapse↔expand because CurrCell
+        // momentarily resolved to null (insideDungeon=false) while the player
+        // stayed in the dungeon landblock — leaking lights via reload storms.
+        // The landblock-hysteresis must hold the collapse.
+        var h = Make();
+        h.State.AddLandblock(MakeLb(0, 7));
+        h.Ctrl.Tick(0, 7, insideDungeon: true);   // collapse
+        h.Loads.Clear();
+        h.Unloads.Clear();
+
+        h.Ctrl.Tick(0, 7, insideDungeon: false);  // CurrCell flicker, same landblock
+
+        Assert.Empty(h.Loads);    // NO full-window reload
+        Assert.Empty(h.Unloads);  // only the center is resident → nothing to sweep
+    }
+
     [Fact]
     public void ExitsDungeon_RebuildsFullWindow_UnloadsStaleDungeonLandblock()
     {