feat(render): Phase 3 (Step A) — outdoor-root seeds full-screen OutsideView

Render unification cutover, Step A (additive, behavior-neutral until Step B). When PortalVisibilityBuilder.Build roots at the synthetic outdoor node, seed OutsideView with the full-screen NDC quad so ClipFrameAssembler yields a full-screen OutsideView slice and DrawInside's DrawLandscapeThroughOutsideView draws terrain/sky/scenery/weather as the node's shell — the same callback that already draws the doorway slice for an interior root looking out.

Keyed on a new explicit LoadedCell.IsOutdoorNode flag (set by OutdoorCellNode.Build), NOT a cell-id heuristic: production EnvCell ids are >= 0x100 but test fixtures use low interior ids, so an id test misfired on 4 existing PortalVisibilityBuilderTests.

Nothing roots at the node until Step B, so this is behavior-neutral. Tests: App 216/0 (2 new UnifiedFloodTests incl. the spec section 10 pure-outdoor regression guard + 2 OutdoorCellNode flag assertions).

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

tests/AcDream.App.Tests/Rendering/OutdoorCellNodeTests.cs

@@ -33,6 +33,7 @@ public class OutdoorCellNodeTests
 
         Assert.Equal(outdoorId, node.CellId);
         Assert.True(node.SeenOutside);
+        Assert.True(node.IsOutdoorNode); // the flag PortalVisibilityBuilder keys the full-screen OutsideView on
         Assert.Equal(Matrix4x4.Identity, node.WorldTransform);
         Assert.Single(node.Portals);
         Assert.Equal((ushort)(0xA9B40170 & 0xFFFF), node.Portals[0].OtherCellId);
@@ -48,5 +49,6 @@ public class OutdoorCellNodeTests
         var node = OutdoorCellNode.Build(0xA9B40031, System.Array.Empty<LoadedCell>());
         Assert.Empty(node.Portals);
         Assert.True(node.SeenOutside);
+        Assert.True(node.IsOutdoorNode);
     }
 }