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>

src/AcDream.App/Rendering/GameWindow.cs

@@ -5985,13 +5985,24 @@ public sealed class GameWindow : IDisposable
             && _playerController.State == AcDream.App.Input.PlayerState.PortalSpace
             && update.Guid == _playerServerGuid)
         {
-            // Compute old landblock coords from controller position (using the
-            // current streaming origin as the reference center).
             var oldPos = _playerController.Position;
-            int oldLbX = _liveCenterX + (int)System.Math.Floor(oldPos.X / 192f);
-            int oldLbY = _liveCenterY + (int)System.Math.Floor(oldPos.Y / 192f);
+            uint streamingOriginLandblockId =
+                AcDream.App.Streaming.StreamingRegion.EncodeLandblockId(
+                    _liveCenterX, _liveCenterY);
 
-            bool differentLandblock = (lbX != oldLbX || lbY != oldLbY);
+            // Retail preserves the full Position (objcell_id + Frame) through
+            // SmartBox::TeleportPlayer @ 0x00453910. Dungeon EnvCells can have
+            // negative local frame origins, so XYZ cannot identify the source
+            // landblock (#215). Compare the controller's authoritative current
+            // cell with the received destination cell instead.
+            var landblockTransition =
+                AcDream.App.Streaming.TeleportLandblockTransition.Classify(
+                    _playerController.CellId,
+                    p.LandblockId,
+                    streamingOriginLandblockId);
+            int oldLbX = (int)((landblockTransition.SourceLandblockId >> 24) & 0xFFu);
+            int oldLbY = (int)((landblockTransition.SourceLandblockId >> 16) & 0xFFu);
+            bool differentLandblock = landblockTransition.CrossesLandblock;
 
             Console.WriteLine(
                 $"live: teleport arrival — old lb=({oldLbX},{oldLbY}) " +
@@ -6013,9 +6024,7 @@ public sealed class GameWindow : IDisposable
                 // position (Resolve's NO-LANDBLOCK verbatim branch, :605) until the
                 // destination streams in. Only the offset-(0,0) center can collide with the
                 // destination-local position, so removing it alone is sufficient.
-                uint staleCenterId = AcDream.App.Streaming.StreamingRegion.EncodeLandblockId(
-                    _liveCenterX, _liveCenterY);
-                _physicsEngine.RemoveLandblock(staleCenterId);
+                _physicsEngine.RemoveLandblock(streamingOriginLandblockId);
 
                 // Recenter the streaming controller on the new landblock NOW (kick
                 // off the dungeon load). After recentering, the destination is

src/AcDream.App/Streaming/TeleportLandblockTransition.cs

@@ -0,0 +1,42 @@
+namespace AcDream.App.Streaming;
+
+/// <summary>
+/// Classifies whether a local-player teleport crosses an AC landblock boundary.
+/// Cell identity comes from the complete retail <c>Position</c>, never from its
+/// frame coordinates: dungeon EnvCells may have valid negative local origins.
+/// </summary>
+/// <remarks>
+/// Retail carries <c>Position::objcell_id</c> through
+/// <c>SmartBox::TeleportPlayer</c> (0x00453910) into
+/// <c>CPhysicsObj::SetPositionSimple</c> (0x005162B0). acdream's asynchronous
+/// streaming layer needs this extra comparison, but preserves that identity rule.
+/// See <c>docs/research/2026-07-13-same-dungeon-respawn-landblock-identity-pseudocode.md</c>.
+/// </remarks>
+internal readonly record struct TeleportLandblockTransition(
+    uint SourceLandblockId,
+    uint DestinationLandblockId)
+{
+    public bool CrossesLandblock => SourceLandblockId != DestinationLandblockId;
+
+    /// <summary>
+    /// Build the transition from the player's current cell and the received
+    /// destination cell. A zero source can occur before initial placement; in
+    /// that case the current streaming center is the only valid identity fallback.
+    /// </summary>
+    public static TeleportLandblockTransition Classify(
+        uint sourceCellId,
+        uint destinationCellId,
+        uint currentStreamingCenterLandblockId)
+    {
+        uint sourceLandblockId = sourceCellId != 0
+            ? NormalizeLandblockId(sourceCellId)
+            : NormalizeLandblockId(currentStreamingCenterLandblockId);
+
+        return new TeleportLandblockTransition(
+            sourceLandblockId,
+            NormalizeLandblockId(destinationCellId));
+    }
+
+    private static uint NormalizeLandblockId(uint cellOrLandblockId)
+        => (cellOrLandblockId & 0xFFFF0000u) | 0xFFFFu;
+}