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/RetailPViewRenderer.cs

@@ -333,8 +333,11 @@ public sealed class RetailPViewRenderer
                 passes.FlushLandscapeAlpha();
                 passes.ClearInteriorDepth();
             };
+            // FW4 slice 2: the seals stamp the WALK'S OWN flood cells (see
+            // DrawWalkExitPortalMasks). walkDriver is assigned below, before
+            // any Replay can fire this closure.
             Action drawExitSeals = () =>
-                DrawExitPortalMasks(ctx, passes, pvFrame, clipAssembly, drawableCells);
+                DrawWalkExitPortalMasks(ctx, passes, clipAssembly, walkDriver!);
 
             var leafRenderer = new WalkProductionLeafRenderer(
                 walkExecutor!, ctx, clipAssembly, clearInteriorDepth, drawExitSeals);
@@ -2035,6 +2038,39 @@ public sealed class RetailPViewRenderer
         }
     }
 
+    /// <summary>Campaign FW4 slice 2 — the walk-flood seal draw. Retail's
+    /// <c>PView::DrawCells</c> stamps every exit portal of THE FLOOD'S OWN
+    /// cells (pc:432785-432786, reverse cell_draw_list far→near) — one
+    /// visibility structure decides the flood, the seals, and the terrain
+    /// views alike. The old apparatus's flood misses exit portals at the
+    /// #456 cathedral seam band (its never-drawn panel family), leaving
+    /// aperture depth unsealed after the interior clear; the end-of-frame
+    /// alpha drain (cell-owned emitters — retail's own timing) then
+    /// z-passes across the whole opening (the falls shine-through,
+    /// probe-pinned via ACDREAM_PROBE_WALK_ROOT's phase tags). Per-cell
+    /// slice clips still come from the old assembly where present; a cell
+    /// the old apparatus missed seals unclipped (the depth fan is the exact
+    /// dat aperture polygon and z-tests, so over-coverage is benign).</summary>
+    private void DrawWalkExitPortalMasks(
+        RetailPViewFrameInput ctx,
+        IRetailPViewPassExecutor passes,
+        ClipFrameAssembly clipAssembly,
+        Walk.WalkFrameDriver driver)
+    {
+        List<uint> floodCells = driver.InteriorFloodCells;
+        for (int i = floodCells.Count - 1; i >= 0; i--)
+        {
+            uint cellId = floodCells[i];
+            foreach (var slice in GetCellSlicesOrNoClip(clipAssembly, cellId))
+                passes.DrawExitPortalMask(
+                    ctx,
+                    new RetailPViewCellSliceContext(
+                        cellId,
+                        slice,
+                        NoParticleOwners));
+        }
+    }
+
     private void DrawEnvCellShells(
         IRetailPViewPassExecutor passes,
         PortalVisibilityFrame pvFrame)