testing: add explanatory commentary to solution

src/testing/solution.md

@@ -3,3 +3,27 @@
 ```rust,editable
 {{#include exercise.rs:solution}}
 ```
+
+The solution addresses several bugs in the original implementation:
+
+- **Input Validation:** Whitespace is now handled explicitly, and invalid
+  characters trigger an immediate failure rather than being silently ignored.
+- **Minimum Length:** The code now correctly enforces the requirement for at
+  least two digits.
+- **Test Coverage:** Added tests for edge cases like empty strings, single
+  digits, and invalid characters.
+
+<details>
+
+- **Unicode Safety:** Rust strings are UTF-8, so they cannot be indexed by
+  integer. Using `.chars().rev()` is the idiomatic way to process a string
+  backwards while correctly handling multi-byte characters.
+- **Handling `Option`:** Using `if let Some(digit) = c.to_digit(10)` is the
+  standard way to handle checked conversions, providing a safe way to deal with
+  potential conversion failures without relying on sentinel values.
+- **Iterators vs. Loops:** This algorithm could be expressed as a functional
+  iterator chain using `filter_map`, `enumerate`, and `fold`. However, an
+  imperative loop is often more readable when maintaining multiple pieces of
+  interdependent state like `sum`, `double`, and `digits`.
+
+</details>