testing: add explanatory commentary to solution

src/testing/solution.md

@@ -3,3 +3,26 @@
 ```rust,editable
 {{#include exercise.rs:solution}}
 ```
+
+The solution addresses several bugs in the original implementation:
+
+- **Whitespace:** We specifically check for and ignore whitespace using
+  `c.is_whitespace()`.
+- **Invalid Characters:** We return `false` immediately if we encounter any
+  character that is not a digit and not whitespace. The original code simply
+  ignored them (via `else { continue }`), effectively treating "123a4" as
+  "1234".
+- **Length Check:** We track the number of digits seen (`digits`) and return
+  `false` if fewer than 2 digits are found.
+- **Unit Tests:** We add tests for edge cases: empty strings, strings with only
+  whitespace, single-digit strings, and strings with invalid characters. This
+  ensures the validation logic is robust.
+
+<details>
+
+- Note that `c.to_digit(10)` returns `Some(digit)` if `c` is a base-10 digit,
+  and `None` otherwise.
+- The condition `digits >= 2 && sum % 10 == 0` ensures both requirements are
+  met.
+
+</details>