modules: add explanatory commentary to solution

src/modules/solution.md

@@ -186,3 +186,27 @@ fn main() {
     window.draw();
 }
 ```
+
+- **File Structure:** We use a directory `src/widgets/` for sub-modules and a
+  file `src/widgets.rs` to define the module itself. This is the standard way to
+  structure modules in modern Rust (the 2018 edition and later).
+- **Visibility:** We use `pub` to make `Button`, `Label`, `Window`, and `Widget`
+  accessible from outside the `widgets` module. Fields of structs (like
+  `Label.label`) remain private by default, preserving encapsulation.
+- **Re-exports:** In `src/widgets.rs`, we use `pub use button::Button;`. This
+  re-exports `Button` from the `widgets` module, so users can import it as
+  `widgets::Button` rather than `widgets::button::Button`. This creates a
+  cleaner public API.
+- **Relative Imports:** The sub-modules (like `label.rs`) use
+  `use super::Widget;` to access the `Widget` trait defined in the parent
+  module.
+
+<details>
+
+- Note that we could have also put `mod` declarations in `src/main.rs` directly
+  referencing the files, but grouping them under a `widgets` module is cleaner
+  for a library.
+- Discuss how `mod.rs` (the older style) is also supported but less common in
+  new code. `src/widgets/mod.rs` would be equivalent to `src/widgets.rs`.
+
+</details>