std-types: add explanatory commentary to solution

src/std-types/solution.md

@@ -3,3 +3,21 @@
 ```rust,editable
 {{#include exercise.rs:solution}}
 ```
+
+- **Generic Key Type:** `Counter` is now generic over `T`.
+- **Trait Bounds:** `T` must be `Eq + Hash` to be a key in a `HashMap`. We apply
+  these bounds on the `impl` block so that `HashMap<T, u64>` works.
+- **Entry API:** `self.values.entry(value).or_default()` provides a mutable
+  reference to the value for the key. If the key is missing, it inserts the
+  default value for `u64` (which is `0`) and returns a reference to that. This
+  is concise and efficient (only one hash lookup).
+
+<details>
+
+- Explain that `Eq` implies `PartialEq`. `HashMap` requires full `Eq` to handle
+  keys correctly.
+- `or_default()` works because `u64` implements `Default`.
+- `unwrap_or_default()` works similarly for `Option`: if the key isn't found
+  (`None`), it returns `0`.
+
+</details>