fix(render): S3 chunk 3 re-review follow-ups

- the LandCell event doc states the deferred cross-block batch rule (the
  same-landblock lookahead text was stale);
- the AlphaSubmitMark arm's comment gives the true reason it is not a
  flush point (it only enqueues into the CPU alpha list; the drain leaves
  flush first);
- the pending terrain batch is cleared with the other transient frame
  lists in AbortFrame/BeginFrame;
- CompleteWalkTerrainFrame runs in a finally so a throwing Replay cannot
  leak its accumulated ticks into the next frame's sample;
- CopyRenderableEmittersInCell keeps its own doc comment;
- the outdoor-root LandCell pin now observes one real cell turn after its
  terrain;
- the oh1 landscape contract's stale "whole pre-stage" prose is resolved.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/AcDream.App.Tests/Rendering/Walk/WalkFrameDriverTests.cs

@@ -1360,7 +1360,12 @@ public sealed class WalkFrameDriverTests
         using var fx = new DispatcherFixture();
         var log = new List<string>();
         var leaf = new RecordingLeafRenderer(log);
+        // Every covered bucket but ONE reports no renderable emitter, so the
+        // block's object turns submit nothing (no flush) except bucket
+        // 0xF4180001, whose particle turn is a real submission — it must land
+        // AFTER the coarse cell's own terrain (retail DrawLandCell → DrawSortCell).
         leaf.CellsWithoutEmitters.UnionWith(CoarseLandscapeBuckets(0xF4180000u));
+        leaf.CellsWithoutEmitters.Remove(0xF4180001u);
         var ctx = new TestContext();
         var driver = new WalkFrameDriver(fx.Dispatcher, leaf, new FakeWorldData());
         var walk = new RetailFrameWalk();
@@ -1378,7 +1383,7 @@ public sealed class WalkFrameDriverTests
         driver.EndFrame();
         driver.Replay(draw.Frame, draw.Pass);
 
-        Assert.Equal(new[] { "SKY", "LANDCELL:f4180000:1:0" }, log);
+        Assert.Equal(new[] { "SKY", "LANDCELL:f4180000:1:0", "PARTICLES:f4180001" }, log);
         Assert.Equal(new (uint LandblockId, int SideCellCount, int CellIndex)[] { (0xF4180000u, 1, 0) },
             Assert.Single(leaf.LandCellBatches));
     }