add header doc comments to non-obvious util data structures

include/parse_buffer.h

@@ -3,6 +3,9 @@
 #include <cstdint>
 #include <span>
 
+// Bounds-checked sequential reader for byte streams. consume<T>() returns a
+// typed pointer into the underlying buffer or nullptr on underflow. Used to
+// walk packet headers in order without UB and without copies.
 class parse_buffer {
     const uint8_t* m_data;
     size_t m_remaining;