fix(G.3): pre-collapse dungeon streaming at login/teleport — kill the login FPS ramp (#135)

On login (or teleport) into a dungeon, FPS started ~10 and climbed over ~30 s.
Root cause: the dungeon "collapse" (which shrinks the 25x25 streaming window to
the player's single dungeon landblock — AC dungeons have no neighbours) only
fires once the per-frame `insideDungeon` gate reads true, and that gate keys on
the physics CurrCell, which isn't set until the player is PLACED, which waits for
the dungeon landblock to hydrate. So during the whole hydration window NormalTick
bootstraps the full window — ~24 unrelated ocean-grid neighbour dungeons + their
~19k entities each — and the collapse only mops them up afterward. That mop-up is
the ramp.

Fix: trigger the SAME collapse early, the instant we recenter the streaming center
onto a sealed dungeon cell, before the first NormalTick.

- StreamingController.PreCollapseToDungeon(cx,cy): fires EnterDungeonCollapse
  early (idempotent). The expensive neighbour window is never enqueued.
- GameWindow.IsSealedDungeonCell(cellId): reads the EnvCell dat SeenOutside flag
  (CurrCell is null pre-placement) — the same flag ObjCell.SeenOutside and the
  per-frame gate use, so the early decision matches the eventual one. Distinguishes
  a real dungeon from a cottage/inn interior (SeenOutside → keeps its outdoor
  surround). Excludes the 0xFFFE/0xFFFF structural shell ids so an outdoor spawn id
  can't type-confuse a LandBlock record as an EnvCell.
- Hooks: OnLiveEntitySpawnedLocked (login) + OnLivePositionUpdated (teleport).
- Observer robustness: during a teleport PortalSpace hold the streaming observer
  follows the recentered destination, not the frozen pre-teleport position (which
  could drift >=2 landblocks off and trip ExitDungeonExpand). And
  _lastLivePlayerLandblockId is now filtered to the player guid (resolves the
  Phase A.1 TODO) so a stray NPC UpdatePosition can't drift the login-hold observer
  off the dungeon.

Faithful EARLY trigger of the existing AP-36 collapse mechanism, not a new
workaround — AP-36 amended in the same commit. Adversarially reviewed across
timing / threading / faithfulness lenses; 5 new tests including the real runtime
ordering (Tick bootstraps, then PreCollapse cancels). Core suite green (1463).

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

src/AcDream.App/Streaming/StreamingController.cs

@@ -149,6 +149,37 @@ public sealed class StreamingController
         DrainAndApply();
     }
 
+    /// <summary>
+    /// #135: collapse to a single dungeon landblock IMMEDIATELY, before the first
+    /// <see cref="Tick"/> has a chance to bootstrap the full 25×25 window. Called
+    /// from the login / teleport spawn path the instant the streaming center is
+    /// recentered onto a SEALED dungeon landblock.
+    ///
+    /// <para>The per-frame <c>insideDungeon</c> gate keys on the physics
+    /// <c>CurrCell</c>, which is only set once the player is PLACED — and placement
+    /// waits for the dungeon landblock to hydrate. So for the whole hydration window
+    /// (tens of seconds for a ~200-cell dungeon) the gate reads false and
+    /// <see cref="NormalTick"/> would enqueue the ~24 unrelated ocean-grid neighbor
+    /// dungeons (+ ~19k entities each); the collapse then only mops them up after
+    /// placement. That mop-up is the 10→high FPS ramp users see at a dungeon login.</para>
+    ///
+    /// <para>Pre-collapsing means the EXPENSIVE dungeon-neighbour window is never
+    /// enqueued. On teleport nothing is enqueued at all (this fires before the next
+    /// Tick recenters). On login a brief Holtburg outdoor window may be enqueued by the
+    /// frame-1 NormalTick (before the player's spawn arrives) and is immediately
+    /// cancelled by <c>_clearPendingLoads</c> here — cheap outdoor terrain, not the
+    /// ocean-grid dungeons, and a handful of already-dequeued loads get swept next
+    /// frame. Idempotent: a no-op when already collapsed onto this same landblock, so a
+    /// re-sent spawn or a same-frame double call costs nothing. Render-thread only,
+    /// same as <see cref="Tick"/>.</para>
+    /// </summary>
+    public void PreCollapseToDungeon(int cx, int cy)
+    {
+        uint centerId = StreamingRegion.EncodeLandblockId(cx, cy);
+        if (_collapsed && _collapsedCenter == centerId) return;
+        EnterDungeonCollapse(cx, cy, centerId);
+    }
+
     /// <summary>
     /// Outdoor / building-interior streaming — the original two-tier model.
     /// </summary>