Document the libakgl API with Doxygen

Add file, structure, global, and function documentation across all libakgl-owned headers and sources, including parameter contracts and likely AKERR/AKGL_ERR exceptions. Add a strict Doxyfile and build the generated API documentation in Gitea CI with warnings treated as failures.

Co-authored-by: Codex (GPT-5) <noreply@openai.com>

include/akgl/iterator.h

@@ -1,6 +1,12 @@
+/**
+ * @file iterator.h
+ * @brief Declares the public iterator API.
+ */
+
 #ifndef _AKGL_ITERATOR_H_
 #define _AKGL_ITERATOR_H_
 
+/** @brief Selects operations and an optional layer for actor traversal. */
 typedef struct {
     uint32_t flags;
     uint8_t layerid;