docs(plan): complete landblock build extraction

docs/plans/2026-07-21-gamewindow-slice-5-landblock-presentation.md

@@ -28,10 +28,19 @@ test, and bisectable commit rather than being hidden in a mechanical move.
     and publication. Origin-less or mismatched request-aware work fails before
     it can publish. Three corrected-diff reviews are clean. Release gate: 6,965
     passed / 5 skipped.
-- [ ] B — capture one immutable world-origin snapshot per load and extract the
+- [x] B — capture one immutable world-origin snapshot per load and extract the
   serialized DAT-backed `LandblockBuildFactory`.
-  - Origin capture is complete in A2; B now contains only the DAT-backed
-    factory extraction and its fixture/serialization gates.
+  - Landed in `5af101b2`: `LandblockBuildFactory` now owns the complete
+    serialized worker-side DAT transaction, immutable height-table snapshot,
+    stab/scenery/interior hydration, complete EnvCell build, and physics-DAT
+    bundle. `GameWindow` only constructs the owner and passes its `Build`
+    operation to the streamer; it no longer contains any build helper body.
+    Hermetic tests pin Far/Near/Promote shape, zero-submesh visibility cells,
+    missing-record behavior, the exact supplied DAT gate, non-interleaved full
+    Near transactions, exception recovery, and immutable inputs. The installed
+    Holtburg gate independently derives visibility completeness from
+    `LandBlockInfo.NumCells`. Three corrected-diff reviews are clean. Release
+    gate: 6,979 passed / 5 skipped; `GameWindow.cs` is 9,573 lines.
 - [ ] C — extract terrain, visibility, EnvCell, building, and render-pinning
   publication.
 - [ ] D — extract physics/cache/collision publication and cross-landblock