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>

src/AcDream.App/Rendering/OutdoorCellNode.cs

@@ -20,6 +20,7 @@ public static class OutdoorCellNode
         {
             CellId = outdoorCellId,
             SeenOutside = true,
+            IsOutdoorNode = true,
             WorldTransform = Matrix4x4.Identity,
             InverseWorldTransform = Matrix4x4.Identity,
         };