smart-pointers: add explanatory commentary to solution

src/smart-pointers/solution.md

@@ -3,3 +3,27 @@
 ```rust,editable
 {{#include exercise.rs:solution}}
 ```
+
+- **Recursive Types with `Box`:** `Node` refers to `Subtree`, which refers to
+  `Node` (via `Box`). `Box<Node<T>>` has a fixed size (pointer size), which
+  breaks the cycle of infinite size that would occur if `Node` directly
+  contained other `Node`s.
+- **Nullability with `Option`:** We use `Option<Box<Node<T>>>` to represent a
+  potentially empty subtree. Rust does not have null pointers; `Option` is the
+  standard way to express absence.
+- **Newtype Pattern:** `Subtree` wraps `Option<Box<Node<T>>>`. This allows us to
+  define methods like `insert` and `len` directly on `Subtree`, keeping the
+  `Node` logic simple. It abstracts away the implementation detail that an empty
+  tree is `None`.
+- **References in `match`:** Notice `match &self.0` and `match &mut self.0`. We
+  match on references to the inner `Option` so that we can inspect or modify the
+  tree without taking ownership of the nodes (which would destroy the tree).
+
+<details>
+
+- Discuss why `Box` is needed. Without it, `sizeof(Node)` would depend on
+  `sizeof(Node)`, which is impossible to resolve.
+- This is a standard BST implementation. Since it's not balanced, performance
+  can degrade to O(N) if elements are inserted in sorted order.
+
+</details>