fix(render): FW4 slice 2 - exit seals stamp the walk''s own flood

The cathedral falls shine-through (owner report at the #456 terrace
seam, toggling with the camera''s side of the seam): the probe''s new
phase tags pinned the falls emitters to the cell-owners route
submitting POST-depth-clear on interior roots - retail''s own timing,
safe only because seals own the aperture depth by the end-of-frame
alpha drain. Our seal SET still came from the old apparatus''s flood,
which misses exit portals at the #456 seam band (its never-drawn panel
family), so the drain z-passed across the unsealed opening.

DrawWalkExitPortalMasks now stamps the walk''s own ordered interior
flood (WalkFrameDriver.InteriorFloodCells, captured at
OnInteriorFloodDrawTurn - retail PView::DrawCells pc:432785-432786),
keeping the old assembly''s per-cell slice clips where present and
sealing unclipped where the old apparatus had no entry (benign: the
exact dat aperture polygon, z-tested). The legacy path keeps the old
seal draw. Also: ACDREAM_PROBE_WALK_ROOT gains [walk-part]/[walk-alpha]
phase-tagged submission/drain lines (the apparatus that pinned this).

Hermetic 6,762/0, Walk lane 213/1, InstalledDat walk conformance 40/1.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.App/Rendering/Walk/WalkFrameDriver.cs

@@ -386,6 +386,17 @@ internal sealed class WalkFrameDriver : IWalkEventSink
     // walk pass dedicated to nothing but set-gathering.
     internal HashSet<uint> VisitedCells { get; } = new();
 
+    /// <summary>FW4 slice 2: the interior root's ORDERED flood cell list,
+    /// exactly as retail's <c>PView::DrawCells</c> iterates it for the
+    /// exit-portal seals (pc:432785-432786) — captured at
+    /// <see cref="IWalkEventSink.OnInteriorFloodDrawTurn"/>. Empty for an
+    /// outdoor-rooted frame. The production seal draw iterates THIS list
+    /// (the walk's own flood), not the old visibility apparatus's — at the
+    /// #456 seam band the old builder's flood misses exit portals the walk
+    /// provably floods, leaving unsealed aperture depth the end-of-frame
+    /// alpha drain splats through (the cathedral falls shine-through).</summary>
+    internal List<uint> InteriorFloodCells { get; } = new();
+
     internal List<WalkBuilding> VisitedBuildings { get; } = new();
 
     internal HashSet<uint> VisitedLandscapeCellIds { get; } = new();
@@ -498,6 +509,7 @@ internal sealed class WalkFrameDriver : IWalkEventSink
         VisitedCells.Clear();
         VisitedBuildings.Clear();
         VisitedLandscapeCellIds.Clear();
+        InteriorFloodCells.Clear();
     }
 
     /// <summary>Records the final segment mark (plan §FW3.2b-1's "at frame
@@ -691,6 +703,13 @@ internal sealed class WalkFrameDriver : IWalkEventSink
         MarkIfGrown();
         _events.Add(WalkFrameEvent.ExitSeals());
 
+        // FW4 slice 2: retain the ordered flood for the seal draw (the
+        // DrawExitSeals leaf runs at Replay, when Collect has long filled
+        // this) — see the property's own doc comment.
+        InteriorFloodCells.Clear();
+        for (int i = 0; i < cells.Count; i++)
+            InteriorFloodCells.Add(cells[i]);
+
         for (int i = 0; i < cells.Count; i++)
             EmitCellTurn(WalkDrawStage.CellStatic, cells[i]);
     }