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/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;
+}