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/Streaming/DungeonStreamingGate.cs

@@ -0,0 +1,51 @@
+namespace AcDream.App.Streaming;
+
+/// <summary>Result of the per-frame dungeon streaming-gate decision.</summary>
+/// <param name="InsideDungeon">Passed to <see cref="StreamingController.Tick"/> — collapse
+/// streaming to the single dungeon landblock.</param>
+/// <param name="ObserverLandblockKey">When non-null, override the streaming observer to this
+/// landblock key (the cell id's high 16 bits, 0xXXYY). Null leaves the caller's observer as-is.</param>
+public readonly record struct DungeonGateResult(bool InsideDungeon, uint? ObserverLandblockKey);
+
+/// <summary>
+/// AP-36: the dungeon streaming gate (#133 FPS). When the player stands in a SEALED
+/// EnvCell (an indoor cell that doesn't see outside), streaming collapses to the single
+/// dungeon landblock — AC dungeons have no adjacent landblocks, so the normal 25×25
+/// window would pull in ~129 unrelated ocean-grid dungeons. The trigger is the player's
+/// CURRENT cell (<c>CellGraph.CurrCell</c>, set the moment the player is placed), and the
+/// observer is pinned to that cell's OWN landblock (the cell id high 16 bits) because a
+/// dungeon's EnvCells sit at arbitrary ocean-grid world coords with negative local offsets.
+///
+/// <para>Extracted from <c>GameWindow.OnUpdate</c> as a pure function so the
+/// teleport-hold rule (below) is unit-testable without the GL/dat/network stack.</para>
+/// </summary>
+public static class DungeonStreamingGate
+{
+    /// <summary>
+    /// Decide the streaming gate from the player's current cell.
+    /// </summary>
+    /// <param name="isTeleportHold">True while a teleport arrival is held (the controller is in
+    /// PortalSpace): the player is NOT yet placed, so <paramref name="currCellId"/> is the frozen
+    /// SOURCE cell, not where the player is going.</param>
+    /// <param name="currCellIsSealedDungeon"><c>CurrCell is EnvCell &amp;&amp; !SeenOutside</c>.</param>
+    /// <param name="currCellId">The current cell id (0xXXYYNNNN).</param>
+    public static DungeonGateResult Compute(
+        bool isTeleportHold, bool currCellIsSealedDungeon, uint currCellId)
+    {
+        // #145/#138: during a teleport hold the player is NOT yet placed, so CurrCell is
+        // the frozen SOURCE cell — where the player IS, not where they're going. Streaming
+        // must follow the DESTINATION, which the PortalSpace observer pin already does, so
+        // the source-cell gate is suppressed. Otherwise a teleport OUT of a dungeon keeps
+        // streaming collapsed on the source dungeon (CurrCell still sealed) → the outdoor
+        // destination never hydrates → TeleportArrivalReadiness holds 600 frames → force-
+        // snap to ocean. A teleport INTO a dungeon is handled explicitly upstream by
+        // StreamingController.PreCollapseToDungeon (and the controller's _collapsed latch
+        // holds it through the hold), so suppressing the gate here doesn't regress it.
+        if (isTeleportHold)
+            return new DungeonGateResult(false, null);
+
+        if (currCellIsSealedDungeon)
+            return new DungeonGateResult(true, currCellId >> 16);
+        return new DungeonGateResult(false, null);
+    }
+}