memory-management: add explanatory commentary to solution

src/memory-management/solution.md

@@ -3,3 +3,27 @@
 ```rust,editable
 {{#include exercise.rs:solution}}
 ```
+
+The solution demonstrates ownership transfer and flexible API design:
+
+- **Consuming Builder:** Methods take `mut self` and return `Self`. This pattern
+  transfers ownership into the method, modifies the state, and returns it,
+  enabling fluid method chaining.
+- **`impl Into<String>`:** This trait bound allows callers to pass either `&str`
+  or `String`, with `into()` performing the conversion to an owned string only
+  when necessary.
+- **Cloning in `as_dependency`:** Since `as_dependency` takes `&self`, it cannot
+  move fields out of the `Package`. We must `clone()` the strings to create an
+  owned `Dependency`.
+
+<details>
+
+- **Why `mut self`?** A consuming builder is idiomatic for one-off construction.
+  If we used `&mut self`, the methods would return `&mut Self`, and the final
+  `.build()` call would still need to consume the builder or clone the entire
+  state.
+- **Inner Representation:** `PackageBuilder` uses a tuple struct to wrap
+  `Package`, providing a clean interface that hides the inner data structure
+  during construction.
+
+</details>