fix(teleport): preserve dungeon cell identity on respawn (#215)

Classify teleport landblock transitions from the source and destination Position cell IDs instead of render-space coordinates. This keeps same-dungeon death respawns on their resident floor physics while preserving the existing true cross-landblock streaming path.

Co-Authored-By: Codex <noreply@openai.com>

tests/AcDream.App.Tests/Streaming/TeleportLandblockTransitionTests.cs

@@ -0,0 +1,63 @@
+using AcDream.App.Streaming;
+
+namespace AcDream.App.Tests.Streaming;
+
+public sealed class TeleportLandblockTransitionTests
+{
+    [Fact]
+    public void SameDungeonRespawn_WithNegativeLocalCoordinate_DoesNotCrossLandblock()
+    {
+        const uint cell = 0x8C0401ADu;
+
+        // The removed implementation inferred the source from render-space XYZ.
+        // A valid dungeon Y of -30.412 m floors into the previous grid row even
+        // though the authoritative source cell still belongs to 0x8C04.
+        int legacyInferredLandblockY = 4 + (int)Math.Floor(-30.412f / 192f);
+        Assert.Equal(3, legacyInferredLandblockY);
+
+        var transition = TeleportLandblockTransition.Classify(
+            sourceCellId: cell,
+            destinationCellId: cell,
+            currentStreamingCenterLandblockId: 0x8C04FFFFu);
+
+        Assert.False(transition.CrossesLandblock);
+        Assert.Equal(0x8C04FFFFu, transition.SourceLandblockId);
+        Assert.Equal(0x8C04FFFFu, transition.DestinationLandblockId);
+    }
+
+    [Fact]
+    public void DifferentEnvCells_InSameLandblock_DoNotCrossLandblock()
+    {
+        var transition = TeleportLandblockTransition.Classify(
+            sourceCellId: 0x8C0401ADu,
+            destinationCellId: 0x8C040145u,
+            currentStreamingCenterLandblockId: 0x8C04FFFFu);
+
+        Assert.False(transition.CrossesLandblock);
+    }
+
+    [Fact]
+    public void DifferentPositionCellLandblocks_CrossLandblock()
+    {
+        var transition = TeleportLandblockTransition.Classify(
+            sourceCellId: 0x8C0401ADu,
+            destinationCellId: 0xA9B40022u,
+            currentStreamingCenterLandblockId: 0x8C04FFFFu);
+
+        Assert.True(transition.CrossesLandblock);
+        Assert.Equal(0x8C04FFFFu, transition.SourceLandblockId);
+        Assert.Equal(0xA9B4FFFFu, transition.DestinationLandblockId);
+    }
+
+    [Fact]
+    public void UnplacedSource_UsesStreamingCenterWithoutCoordinateInference()
+    {
+        var transition = TeleportLandblockTransition.Classify(
+            sourceCellId: 0,
+            destinationCellId: 0x8C0401ADu,
+            currentStreamingCenterLandblockId: 0x8C04FFFFu);
+
+        Assert.False(transition.CrossesLandblock);
+        Assert.Equal(0x8C04FFFFu, transition.SourceLandblockId);
+    }
+}