Update reactive scoping docs

todo.md

@@ -9,26 +9,4 @@
   - batch
 - optimize `indexes()` double-diffing
 - improve crash course, some sections feel like information dumps
-- reactive scopes
-  - define behavior of creating a reactive scope within a non-root reactive scope
-    - i.e. an effect within an effect
-    - error on attempt for now
-  - define behavior of destruction of a node with a child, where the node and
-    its child are in separate, non-nested root reactive scopes.
-    - should destruction of the parent also destroy the child?
-    - or should destruction of the parent silently disconnect the child, without
-      invoking the child's cleanups?
-
-    ```mermaid
-    graph LR
-
-    subgraph root1
-        parent
-    end
-
-    subgraph root2
-        child
-    end
-
-    parent --> child
-    ```
+- cleanup source and tests