add header doc comments to non-obvious util data structures

include/callback_list.h

@@ -1,6 +1,9 @@
 #pragma once
 #include <utility>
 
+// Fixed-capacity doubly-linked list with an embedded node array and intrusive
+// free list. Used for callback registries (frame callbacks, timers) needing
+// O(1) insert/remove with stable handles, without heap in IRQ-adjacent paths.
 template <typename T, int N>
 struct callback_list {
     struct node {