lifetimes: add explanatory commentary to solution

This commentary, written by Gemini, focuses on aspects of the solution that differ from the baseline languages (C/Java/Python), highlighting Rust-specific idioms and concepts.

Author: mgeisler

src/lifetimes/solution.md

@@ -3,3 +3,25 @@
 ```rust,editable
 {{#include exercise.rs:solution}}
 ```
+
+- **Zero-Copy Parsing:** This solution demonstrates zero-copy parsing. The `Person`
+  and `PhoneNumber` structs borrow string slices (`&'a str`) directly from the
+  input byte array (`&'a [u8]`). No new strings are allocated for the field values.
+- **Lifetimes:** The lifetime `'a` ensures that the returned `Person` cannot
+  outlive the input data buffer. If the buffer is dropped, the `Person` becomes
+  invalid, which the compiler enforces.
+- **Slicing:** We use `split_at` to divide the input buffer into the data for the
+  current field and the remainder. This is efficient as it only manipulates
+  pointers and lengths, not the data itself.
+- **Recursion:** The parsing is recursive. `Person` contains `PhoneNumber`s, which
+  are themselves parsed by calling `parse_message` on the bytes contained within
+  the `Len` wire type.
+
+<details>
+
+- Note that `parse_message` requires `T` to implement `Default` so it can start
+  with an empty message and fill it in.
+- The `WireType` enum and `parse_varint` function handle the low-level encoding
+  details, allowing the main logic to focus on structure.
+
+</details>