fix(G.3): pin dungeon collapse to the cell's landblock, not the position-derived one (#133)

"The dungeon is broken" — the collapse was unloading the REAL dungeon. A dungeon's
EnvCells sit at arbitrary "ocean" world coords with negative cell-local Y (snap
showed pos=(58.9,-69.6) in cell 0x00070133), so the observer landblock
_liveCenterY + floor(pp.Y/192) = 7 + floor(-69.6/192) = 7 + (-1) = 6 lands one row
off. The collapse pinned to 0x0006 and unloaded 0x0007 — the real dungeon — which
nulled CurrCell (the cell no longer existed) and left the player floating in
outdoor-lit empty space (lb 1/1 @ ~1585 fps, but the wrong landblock). This is the
Bug-A negative-local-coordinate class.

Fix: when inside a dungeon, pin the collapse to the cell's OWN landblock
(CurrCell.Id >> 16), never the position-derived observer landblock — the cell id is
the authoritative landblock for ocean-placed dungeon geometry.

Also hardened the hysteresis so a transient CurrCell flicker can't thrash:
- Re-collapse when insideDungeon at a DIFFERENT landblock (multi-landblock dungeon).
- Expand only on a DISTANT move (Chebyshev > 1) — a real exit teleports far from the
  ocean-grid block; the off-by-one flicker is always an ADJACENT (±1) landblock, so
  it now HOLDS the collapse instead of expanding.
- SweepCollapsed always preserves _collapsedCenter (the true dungeon landblock),
  never the per-frame observer landblock.

Build green; 59 streaming tests green (flicker regression test updated to the
realistic adjacent off-by-one).

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -6892,10 +6892,23 @@ public sealed class GameWindow : IDisposable
             // and keep their surrounding terrain.
             // Mirrors the playerInsideCell computation below (CurrCell → registry
             // LoadedCell.SeenOutside): true only for a sealed indoor cell.
-            bool insideDungeon =
-                _physicsEngine.DataCache?.CellGraph.CurrCell is AcDream.Core.World.Cells.EnvCell pcEnv
+            bool insideDungeon = false;
+            if (_physicsEngine.DataCache?.CellGraph.CurrCell is AcDream.Core.World.Cells.EnvCell pcEnv
                 && _cellVisibility.TryGetCell(pcEnv.Id, out var pcReg)
-                && pcReg is { SeenOutside: false };
+                && pcReg is { SeenOutside: false })
+            {
+                insideDungeon = true;
+                // Pin the collapse to the cell's OWN landblock (cell id high 16 bits),
+                // NOT the position-derived observer landblock. A dungeon's EnvCells sit
+                // at arbitrary world coords (the "ocean" placement) with negative local
+                // offsets, so floor(pp.Y/192) lands one landblock off — which collapses
+                // onto the WRONG landblock and unloads the real dungeon, nulling CurrCell
+                // and breaking the render (the Bug-A coordinate class). The cell id is the
+                // authoritative landblock.
+                uint cellLb = pcEnv.Id >> 16;
+                observerCx = (int)((cellLb >> 8) & 0xFFu);
+                observerCy = (int)(cellLb & 0xFFu);
+            }
             _streamingController.Tick(observerCx, observerCy, insideDungeon);
 
             // Re-inject persistent entities rescued from unloaded landblocks

src/AcDream.App/Streaming/StreamingController.cs

@@ -120,13 +120,22 @@ public sealed class StreamingController
 
         if (_collapsed)
         {
-            // Hysteresis: stay collapsed while the player remains in the dungeon
-            // landblock, regardless of CurrCell flicker. Expand only on an actual
-            // landblock change (the player left through a portal / was teleported).
-            if (centerId != _collapsedCenter)
+            // Hysteresis. Cases:
+            //  - Still in the SAME dungeon landblock → hold (sweep stragglers).
+            //  - In a DIFFERENT dungeon cell (multi-landblock dungeon / new dungeon)
+            //    → re-collapse onto it.
+            //  - CurrCell flickered null but the player hasn't gone anywhere: the
+            //    observer landblock reverts to the position-derived value, which for a
+            //    dungeon is only ever the ADJACENT off-by-one landblock (negative cell-
+            //    local Y). Hold — never expand on an adjacent flicker.
+            //  - Genuinely left to a DISTANT landblock (portal/teleport out, always far
+            //    from the ocean-grid dungeon block) → expand.
+            if (insideDungeon && centerId != _collapsedCenter)
+                EnterDungeonCollapse(observerCx, observerCy, centerId);
+            else if (!insideDungeon && ChebyshevLandblocks(centerId, _collapsedCenter) > 1)
                 ExitDungeonExpand(observerCx, observerCy);
             else
-                SweepCollapsed(centerId);
+                SweepCollapsed();
         }
         else if (insideDungeon)
         {
@@ -200,10 +209,20 @@ public sealed class StreamingController
     /// effect. At steady state only the dungeon landblock is resident, so this
     /// is a no-op.
     /// </summary>
-    private void SweepCollapsed(uint centerId)
+    private void SweepCollapsed()
     {
+        // Always preserve the true dungeon landblock (_collapsedCenter), never the
+        // per-frame observer landblock — a CurrCell flicker must not unload the dungeon.
         foreach (var id in _state.LoadedLandblockIds)
-            if (id != centerId) _enqueueUnload(id);
+            if (id != _collapsedCenter) _enqueueUnload(id);
+    }
+
+    /// <summary>Chebyshev distance in landblock cells between two landblock ids.</summary>
+    private static int ChebyshevLandblocks(uint a, uint b)
+    {
+        int ax = (int)((a >> 24) & 0xFFu), ay = (int)((a >> 16) & 0xFFu);
+        int bx = (int)((b >> 24) & 0xFFu), by = (int)((b >> 16) & 0xFFu);
+        return Math.Max(Math.Abs(ax - bx), Math.Abs(ay - by));
     }
 
     /// <summary>

tests/AcDream.Core.Tests/Streaming/StreamingControllerDungeonGateTests.cs

@@ -110,22 +110,24 @@ public class StreamingControllerDungeonGateTests
     }
 
     [Fact]
-    public void Collapsed_CurrCellFlickersToNull_SameLandblock_DoesNotExpand()
+    public void Collapsed_CurrCellFlickersToAdjacentOffByOne_DoesNotExpand()
     {
-        // Regression: the live run thrashed collapse↔expand because CurrCell
-        // momentarily resolved to null (insideDungeon=false) while the player
-        // stayed in the dungeon landblock — leaking lights via reload storms.
-        // The landblock-hysteresis must hold the collapse.
+        // Regression: the live run broke because a dungeon cell's negative local-Y
+        // makes the position-derived observer landblock land one row off (0,7→0,6).
+        // When CurrCell flickers null mid-frame, GameWindow stops overriding to the
+        // cell landblock and passes that adjacent (0,6). The Chebyshev>1 guard must
+        // treat that as a flicker and HOLD — never expand (which would unload the
+        // real dungeon and re-stream the 25×25 neighbor window).
         var h = Make();
         h.State.AddLandblock(MakeLb(0, 7));
-        h.Ctrl.Tick(0, 7, insideDungeon: true);   // collapse
+        h.Ctrl.Tick(0, 7, insideDungeon: true);   // collapse onto the dungeon (0,7)
         h.Loads.Clear();
         h.Unloads.Clear();
 
-        h.Ctrl.Tick(0, 7, insideDungeon: false);  // CurrCell flicker, same landblock
+        h.Ctrl.Tick(0, 6, insideDungeon: false);  // flicker → adjacent off-by-one
 
         Assert.Empty(h.Loads);    // NO full-window reload
-        Assert.Empty(h.Unloads);  // only the center is resident → nothing to sweep
+        Assert.Empty(h.Unloads);  // dungeon (0,7) preserved; nothing else resident
     }
 
     [Fact]