fix(streaming): #145 — teleport re-use via server-authoritative placement

Portals only worked once per session: teleporting OUT of a dungeon
mis-rooted the player into the SOURCE dungeon's coordinate frame, so every
move was sent dungeon-framed and ACE rejected it ("failed transition") —
the player couldn't move, never reached a portal, and the world wouldn't
re-render (only skybox).

Root cause: acdream's streaming-relative frame recenters on teleport, but
resident physics landblocks keep their load-time world-offset. After
recentering onto the outdoor destination, the collapsed source dungeon
(offset 0,0 as the prior center) and the destination (offset 0,0 as the
new center) overlap, and the Z-agnostic outdoor cell-snap returns the
dungeon for both the arrival placement and every per-frame resolve.

Fix (server-authoritative teleport placement):
- Drop the stale source center landblock from physics at the teleport
  recenter (GameWindow.OnLivePositionUpdated) so the resolve falls through
  to the server position (Resolve NO-LANDBLOCK verbatim) until the
  destination streams in.
- Place outdoor teleports immediately (TeleportArrivalRules) — holding is
  futile because streaming does not progress during a PortalSpace hold.
- Clear a dangling CellGraph.CurrCell when its landblock is removed
  (PhysicsEngine.RemoveLandblock) — otherwise the dungeon-streaming gate
  keeps streaming collapsed onto the gone dungeon (only skybox renders).

Keeps DungeonStreamingGate (gate suppression during the hold). Indoor
(dungeon-entry) placement is unchanged (cell-keyed, IsSpawnCellReady).

User-verified: in->out->re-enter works repeatedly, no ACE errors, world
renders. Remaining facets (server objects + own avatar not rendering after
a teleport-out) are entity render/lifecycle — split to #138.

Registers AP-36 + AD-2 updated. New: DungeonStreamingGate (+4 tests),
TeleportArrivalRules (+4 tests). Build + 2727 tests green.

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

src/AcDream.App/World/TeleportArrivalController.cs

@@ -103,3 +103,42 @@ public sealed class TeleportArrivalController
         _place(_destPos, _destCell, forced);
     }
 }
+
+/// <summary>
+/// Pure readiness decision for a teleport arrival, factored out of
+/// <c>GameWindow.TeleportArrivalReadiness</c> for testing.
+/// </summary>
+public static class TeleportArrivalRules
+{
+    /// <summary>
+    /// Decide whether a held teleport arrival can place now.
+    /// </summary>
+    /// <param name="claimUnhydratable">The destination cell can never hydrate.</param>
+    /// <param name="indoor">Destination is an indoor cell (cell index ≥ 0x0100).</param>
+    /// <param name="indoorCellReady">For an indoor destination, the EnvCell floor has hydrated.</param>
+    public static ArrivalReadiness Decide(
+        bool claimUnhydratable,
+        bool indoor,
+        bool indoorCellReady)
+    {
+        if (claimUnhydratable)
+            return ArrivalReadiness.Impossible;
+
+        // Indoor (sealed dungeon / building interior): gate on the EnvCell floor hydrating,
+        // exactly as #135. The dungeon-IN path pre-collapses + waits for the cell struct, and
+        // its placement validates the specific claimed cell (the indoor resolve is cell-keyed,
+        // not the grid-snap), so it is unaffected by the overlap that broke teleport-OUT.
+        if (indoor)
+            return indoorCellReady ? ArrivalReadiness.Ready : ArrivalReadiness.NotReady;
+
+        // Outdoor: place IMMEDIATELY on the server-authoritative position. #145/#138 — holding
+        // for the destination to stream in is futile: streaming does NOT progress while the
+        // player is held in PortalSpace (the destination only loads once placement flips the
+        // state to InWorld). The stale source center landblock is dropped at recenter (see
+        // GameWindow.OnLivePositionUpdated), so the arrival resolve falls through to the
+        // server-authoritative cell/position (Resolve NO-LANDBLOCK verbatim) and the player is
+        // placed grounded there; streaming then loads the destination and the per-frame resolve
+        // grounds onto it. This trusts the server's teleport position — retail-faithful.
+        return ArrivalReadiness.Ready;
+    }
+}