remove: old scrollbox-tabbar.org (all prose distributed to per-module orgs)

The combined org file had no unique content — all prose and code were
already in scrollbox.org, tabbar.org, and container-package.org. The
old file's code blocks had the pre-bugfix render/draw-scrollbars
versions and all had :tangle no.

Also update README.org and ARCHITECTURE.org references from
scrollbox-tabbar.org to the individual org files.

docs/ARCHITECTURE.org

@@ -285,7 +285,9 @@ reads terminal background color at startup.
   │   ├── markdown-renderer.org
   │   ├── modern-backend.org
   │   ├── mouse.org
-  │   ├── scrollbox-tabbar.org
+  │   ├── scrollbox.org
+  │   ├── tabbar.org
+  │   ├── container-package.org
   │   ├── select.org
   │   ├── slot.org
   │   └── text-input.org